The first time I read Middlemarch I was 17 years old and studying for my university entrance exams. Every Sunday some classmates and I went for a few hours to the home of a retired teacher of English literature, who had been enlisted by my grammar school to tutor us. She lived in a small village on the outskirts of the seaside town in Dorset where I grew up. In the summer, my hometown was filled with holidaymakers eating fish and chips and pitching striped canvas windbreaks on the beach. In the winter it was barren, with dreary skies and sea of the same unending grey. In no season did it satisfy my malcontented teenage self. As I sat in the teacher's living room, looking out over hills that were often sodden, grazed by forlorn sheep, the place I came from felt barely less provincial than the town anatomised by George Eliot in the novel that lay on my lap.

I was dying to get away from this landscape. Oxford was the immediate goal, but anywhere would do. My town had no colleges, no theatres and no museums. It appeared to offer no opportunity to live a cultured, intellectual life, which was what I avidly aspired to do, even if I had only a very vague notion of what that might consist of. When I read about Dorothea Brooke – an ardent young gentlewoman who yearns for a more significant existence – I recognised in her imprecise longing a mirror of my own restless, ill-formed ambitions.

This recognition occurred in spite of significant differences between us – Dorothea is a beautiful, well-born heiress, while I grew up in a modest house built in the 1950s, and could count among my Victorian progenitors men and women who worked in houses like that in which Dorothea lives. Dorothea was devout; in those years of Margaret Thatcher's ascendance I had a teenager's reflexive radicalism, and had spent many Saturdays hovering outside Woolworths in the town centre, trying to sell copies of leftwing newspapers to shoppers who could not have been less interested. Dorothea is 19, an age at which it is thought appropriate that she get married. I wasn't sure I ever wanted to get married, and if I did, it was certainly not going to be when I was 19. There was so much more I wanted to do. I just wasn't sure what it was.

But those differences didn't matter. When I was a teenager, Middlemarch – which charts the loves and longings of a number of inhabitants of a fictional Midlands town circa 1830 – seemed to me to be all about the aspirations of youth. Even if the focus of that aspiration had changed, I recognised Dorothea's sense of yearning as akin to my own. I also admired Eliot, from the little I knew about her, who seemed to share Dorothea's yearning for a life other than the one she was born into. Eliot was born Mary Ann Evans in Nuneaton in 1819; she lived with a man to whom she wasn't married, writing her novels under a pseudonym: an inspiring and intriguing example to a young feminist who also wanted to be a writer.

Middlemarch, written almost 100 years before I was born, wasn't the least bit dry or dusty. I couldn't believe how relevant it felt. And I couldn't believe how good it was. In those days, when I felt I was waiting for my life to begin, I used to write long letters to a cousin close to my own age, recounting music listened to, books read. In one letter, I reported getting through Dostoevsky's The Gambler, Fitzgerald's The Great Gatsby and The Beautiful and Damned, Austen's Pride and Prejudice and Northanger Abbey, and Middlemarch, which, I opined airily, "is a brilliant novel and should be read".

I loved Middlemarch, and I loved being the kind of person who loved it. I knew that many critics regarded it as the greatest novel in English literature, and I wanted to be among those who understood why. Reading it, and beginning to comprehend it, was a step on the path to the life I sought. When I went for my interview at Oxford I met with the senior tutor in English at what was to become my college – a forbidding Scotsman possessed of what I only later realised was a magnificently dry sense of humour. His study was furnished with low-slung easy chairs upholstered in mustard-coloured corduroy; one could either perch on a chair's edge or sink into its depths. His study was cold, or I was shivering with nerves – either way, I did not take off my charity-shop coat, made from astrakhan, with a fur collar, as I sat in his easy chair and shifted from one uncomfortable position to another, talking passionately about Middlemarch. Not long afterwards I received a letter saying I had been accepted, and that was the first time Middlemarch changed my life.

Middlemarch got me to Oxford, and I read it again there, too. "Discuss George Eliot's treatment of 'oppressive narrowness' and its effect on her characters," was the essay question I chose to answer in the exams at the end of my first year, where the hard chair and the grand hall of the Examination Schools amounted to my own escape from oppressive narrowness. It was the mid 1980s, and such exotic concepts as critical theory were in the air. Instead of merely reading books I learned to interrogate texts, as if they had committed some criminal malfeasance. Even Middlemarch came in for this treatment: "This incoherent, heterogeneous, 'unreadable,' or nonsynthesisable quality of the text of Middlemarch jeopardises the narrator's effort of totalisation," wrote the critic J Hillis Miller, in a much-borrowed library volume.

After graduating I was ready to escape even further, and moved to New York City. My plan was to stay for a year – I was doing a master's in journalism – and then to return to London to look for a job at a newspaper, equipped with experience that would set me apart from all the other eager would-be journalists. Studying journalism in a classroom, it turned out, was mostly absurd. One instructor, a weary former city reporter, conducted pretend press conferences in which he would impersonate the whiny, petulant mayor of the city, while we asked pretend questions. Another instructor aired her dispiriting opinion in the first class that most of us would end up in PR.

Dorothea and Casaubon. Illustration by Cecilia Lundgren for the Guardian

But I also got a part-time job at a weekly magazine, where I did research for writers, answered phones, did photocopying, and even wrote a few short pieces, learning skills and gaining experience that only a real job – with a real pay cheque – could provide. A few weeks before I was due to move back to England, a full-time job in the magazine's fact-checking department opened up. I was offered it, and decided that I wasn't going to move back home after all. And, apart from one short stint at a newspaper in London in my mid 20s, I have spent my entire adult life on the other side of the Atlantic from my childhood home.

I didn't feel at home in New York – it was too alien for that – but I enjoyed the sense of estrangement it offered. At first I shared a small apartment that had sloping floorboards, exposed-brick walls and an occasional mouse problem four flights above a busy SoHo intersection. On summer nights when it was too hot to sleep I would sit on the fire escape, looking over the water towers on the building opposite and down on the streets below, enjoying the exotic sensation of sultry air on my skin. Where I grew up it was always cool at night, even after the warmest of days. I found the abrasiveness of New York exciting, even glamorous. I liked being able to yell at someone who shoved me on the subway, rather than feeling obliged to fume inwardly. I felt free, independent, exhilarated. And by my mid 20s, I was no longer a fact-checker. I had become a feature writer at the magazine where I worked, writing often-acerbic profiles of cultural figures, which is one way for a young writer to get noticed. I lived in a tiny apartment downtown: the first home I made for myself alone.

When she was not much older than I was George Eliot had been a journalist, too. Having grown up in the provincial towns of Nuneaton and Coventry, Eliot moved in her early 30s to London, becoming the editor in all but name of the Westminster Review. There, she became the anonymous author of a number of devastating critiques of writers and thinkers, including a piece about the poet Edward Young, whom she had revered in her youth but now castigated for moral falseness. She also wrote an essay about romance fiction called "Silly Novels by Lady Novelists" – a title that today would not look amiss as a cheeky headline on an irreverent website. Her critical judgment could be astringent. If one is accustomed to thinking of Eliot as the capaciously generous novelist she became, it's bracing and not a little gratifying to read these essays and realise that she too had a frustrated ferocity it satisfied her to unleash.

In her private letters of that time Eliot could be spiky, defensive in anticipation of attack. "Treating people ill is an infallible sign of special love with me," she wrote to one friend. Her career was taking off: leading intellectuals quickly acknowledged her as their peer, and perhaps even suspected she was their superior. But her personal life was less successful. She had a brief entanglement with John Chapman, the head of the Westminster Review, who also happened to be her landlord. Then friends suspected that her closeness with Herbert Spencer was heading towards marriage; Spencer rejected her, to Eliot's great anguish. Her letters to him, buried for many decades in the British Library, make for painful reading.

Eliot in these years seems a strikingly contemporary figure: a young woman, alone in a city, forging a career and hoping for love. "I can see her now, with her hair over her shoulders, the easy chair half sideways to the fire, her feet over the arms, and a proof in her hands," a friend wrote of her in later years. I can see her, too, because I know this woman. I've been this woman. In my own tiny apartment, I didn't have any bookshelves. My copy of Middlemarch and hundreds of other books were stacked in precarious piles, their orange and black and grey spines abutting each other like bricks in an uneven wall that has been foolishly and irrevocably constructed without foundations. They loomed over my bed: a futon that I rarely folded into a couch. I had little other furniture. A desk was wedged into a closet. In a kitchen that also served as a hallway were a tiny folding table and two chairs. The place was adequate to entertain a single significant visitor, when the occasion arose.

And when I came back to Middlemarch again in my mid 20s – perched on my fire escape overlooking the bustling street, or sunk into my one easy chair, with my feet over the arm – it spoke to me in a different way than it had done when I was 17. Then, I had identified with Dorothea's ardent yearning for a different life. Now, as I embarked on one misbegotten love affair, and then another, the book seemed to be all about the purpose and meaning of marriage. Middlemarch didn't offer instruction, or at least I didn't read it as offering such. If I had, I might not have become successively involved with men seemingly older and seemingly wiser than I was, who occupied that second folding chair, and preoccupied my thoughts. But I could tell that the novel offered illumination, even if I didn't yet know quite how to answer the questions it posed. What constituted a good marriage? What made a bad one? How is one to know whom to build a life's love with?

Eliot shows a number of different marriages, successful ones and ones that are fatally ill founded. At the outset of Middlemarch, Dorothea marries the Reverend Edward Casaubon: a pedantic, dry-as-dust scholar whom she mistakenly imagines to have a great mind. She discovers within a painfully short space of time after their wedding day that he is defensive and proud, and she is appalled by her efforts to achieve physical and emotional intimacy with him. That terrible marriage is followed by another fundamentally flawed match within the community: that of Tertius Lydgate, an ambitious young doctor, newly arrived in Middlemarch, who aspires to make great discoveries, but whose ambitions are fatally curtailed by his marriage to Rosamond Vincy, the town beauty, who is as empty-headed and stubborn as she is pretty and beguiling.

In these two central stories, Middlemarch showed me the different ways in which getting married could constitute a terrible error. In a wonderfully restrained phrase, Eliot writes of "the difficulties of civilisation", that require men and women to marry before they have much more than a passing acquaintance with each other. Even with a lessening of those difficulties in my own time, it still seemed perilously easy to make a mistake. And Middlemarch also showed me what the cost of long-term commitment might be, in the marriage of Nicholas and Harriet Bulstrode. He is an odiously sanctimonious banker who came by his fortune through deceit, and whose eagerness to preserve his good name leads him deep into moral compromise. Mrs Bulstrode's discovery of her husband's wrongdoing, and then her decision not to be among those who turn against him, makes for one of the most moving episodes in the book. Alone in her bedroom, she removes her jewellery, puts on a plain cap, then descends to him, resolved to "espouse his sorrow". What would it be, I wondered, to promise someone you'd do that?

George Eliot and George Henry Lewes met, appropriately enough, in a bookshop. Set among the tobacconists and shirtmakers and leather shops of Burlington Arcade, Jeffs specialised in foreign-language books imported from the continent. The shop was cramped – only nine-feet deep and not much wider. There would barely have been room to turn around, especially if wearing an ample Victorian gown. These days, it's a high-end jewellery shop. I went there one day, looking idly, and in vain, for an emerald-and-diamond ring, like the one that seduces Dorothea with its beauty in the first chapter of Middlemarch.

Less than three years after that introduction, when Eliot was in her 35th year, she and Lewes eloped to Germany; they went on to live together for 24 years. Marriage was impossible: he was already married and divorce was out of the question. The critic Phyllis Rose has written warmly of their union, "The Leweses manage to be as happy together for the 24 years they lived together as any two people I have heard of outside fantasy literature." Elizabeth Hardwick, the American novelist and critic, conjured their companionability with almost concupiscent precision: "working, reading, correcting proofs, travelling, entertaining, receiving and writing letters, planning literary projects, worrying, doubting their powers, experiencing a delicious hypochondria." Hardwick was married to the poet Robert Lowell when she wrote those lines, and in her description of Victorian literary couples – "Before the bright fire at teatime, we can see these high-strung men and women clinging together, their inky fingers touching" – there seems to be a touch of admiring identification.

From left: Patrick Malahide as Casaubon, Juliet Aubrey as Dorothea and Rufus Sewell as Ladislaw in the 1994 BBC adaptation. Photograph: BBC

I'm inclined to join in this celebration of Eliot and Lewes's life together, and to cherish their late love, because it was not until I was 35 that I met the man who became my husband. He is a writer too, and on those days when we are working in different corners of our house, or travelling together for research, or reading one another's work before another editor has seen it, I think I have an inkling of what Eliot and Lewes's life together must have been like. Lowell, in an unrhymed sonnet about Eliot, called her union with Lewes "Victorian England's one true marriage". It would be hard to find a happier model for a writers' marriage than theirs.

Like Lewes, who had three sons from his marriage, whom Eliot helped to support, my husband was the father of three boys when I married him. Eliot is often thought of as being a childless writer; but she wasn't, not really, and any step-parent might recognise him or herself in the words Eliot wrote to a friend shortly before the three boys – who had been away at boarding school, in Switzerland – were to take up residence with her and Lewes in their London home. "I hope my heart will be large enough for all the love that is required of me," she admitted. Eliot's feelings towards the three Lewes boys were complex. Though she cherished the oldest boy, Charles, a great deal, she seems to have been less able to find quite enough love in her heart to enjoy having the younger two around for any length of time. As young men, both were dispatched to colonial adventures in Africa, and both met early deaths. When I read Middlemarch now, in the light of Eliot's experience as a stepmother – and in the light of my own – I am struck by the way it offers variations on the paths that a young man might take: there is Lydgate with his focused ambition, Ladislaw with his erratic, artistic dreams, and Fred, with his wishful expectation of good fortune.

"People may really have in them some vocation which is not quite plain to themselves, may they not?" Dorothea asks at one point, when Casaubon has been describing his young cousin Ladislaw as a dilettante. "They may seem idle and weak because they are growing. We should be very patient with each other, I think." These are words any parent, not just a step-parent, would be wise to remember. In them, I hear Eliot's own, not always successful, efforts towards patience with the three young men who arrived in her life unexpectedly. And I recognise my own obligations to the three young men who came into my life, bringing with them a great deal of joy, and some degree of care, as all children do – as my son, their much younger brother, did, when he was born a year after my husband and I were married. Patience, patience. Middlemarch reminds me that I must grant all my children that, and much more.

"Few women, I fear, have had such reason as I to think the long sad years of youth were worth living for the sake of middle age," Eliot wrote in her diary on the last day of 1857. She was 38, which is an age at which most of us are trying to fend off any admission that middle age is upon us. But in the mid‑19th century a woman was thought matronly at the very least by the time she was in her early 30s, and Eliot had long ceased to think of herself as a young woman when she met George Henry Lewes.

But growing older was not to be dreaded. "One has to spend so many years in learning how to be happy," she wrote to a friend when she was 24. Childhood, she wrote, is full of incomprehensible sorrows, even if it can seem glorious in retrospect. "All this, dear Sara, to prove that we are happier than when we were seven years old, and that we shall be happier when we are 40 than we are now, which I call a comfortable doctrine and one worth trying to believe."

Eliot was happier at 40 than she had been at 25. She had experienced the joy of finding her creative voice, and the satisfaction of having her work well received: her first full-length novel, Adam Bede, which was published in the year she turned 40, was widely acclaimed and avidly read. She was happy in her personal life, and remained so. "In my private lot I am unspeakably happy, loving and beloved," she wrote in her diary at the end of 1870, when she was writing Middlemarch, almost 20 years after she and Lewes had met.

Eliot and Lewes seem to have been one of those enviable pairs who appreciate each other more as they grow older together, for reasons she hinted at in an essay she wrote about Madame de Sablé, the 17th-century salonnière, in the summer of 1854, when she and Lewes had first departed for Germany together. "It is undeniable, that unions formed in the maturity of thought and feeling, and grounded only on inherent fitness and mutual attraction, tended to bring women into more intelligent sympathy with men," she wrote. She makes late-arriving love sound irresistibly romantic – different from the agonies and ecstasies of young love, but no less appealing, and significantly more satisfying.

I, too, find myself happier in my 40s than I was in my 20s, and rereading Middlemarch now helps me understand why. It is a book that is mostly about young people, and when one reads it as a young person it seems to be all about the preoccupations of youth – the romantic dramas and the personal ambitions. But it was a book written by a person well into middle age, and is infused with the wisdom that can only be accrued by the experience of heartbreak and failure.

A compensation of getting older is an increasing ability to see the comedy of human relations, which can be obscured by the tempests of youthful emotion. Even stormy, passionate Will Ladislaw will become a more measured person as he grows older, one with a sense of humour about his own passions. Or so Eliot hints, in her description of him as a middle-aged man, looking back on his desperate non-courtship of Dorothea, explaining why he had not simply thrown himself at Dorothea's feet once Casaubon had died. "He used to say the horrible hue and surface of her crape dress was likely the sufficient controlling force," Eliot writes. In this account I hear an ironical note of self-retrospection, even amusement, as Ladislaw remembers the boundless emotions of first love.

Virginia Woolf famously called Middlemarch "one of the few English novels written for grownup people" – a judgment so often recited that it is rarely examined. But what an interesting choice of phrase it is: "Grownup people" is what children call adults; it is not usually what we adults call ourselves. ("A merry day for the children, who ran and shouted to see if they could top the wind with their voices; and the grownup people, too, were in good spirits, inclined to believe in yet finer days when the wind had fallen," Eliot writes in Adam Bede.) One thing that Woolf meant, I think, was to suggest that the novel does not depend on the tidy resolution that so many children's stories provide. The ending is happy for some characters and disappointing for others. And for this reader, at least, the conclusion of the novel has always been irresistibly melancholy.

But in revisiting Middlemarch in middle age, the melancholy I experience in reading its final pages is augmented by a strange glimmer of hope, even optimism. I see in it now what I could not see as a young person: that wisdom is always being acquired, and is never fully accomplished; that love can arrive in unimagined ways, and may be found where we least expect it. "Every limit is a beginning as well as an ending," Eliot writes at the end of Middlemarch. Our own limited lives might also contain the possibility of renewal. Only a child believes a grownup has stopped growing.