On Wednesday 24 September 1975, I was at home in Nottingham, off school with a “tummy upset”. “Had a bowl of cornflakes this morning,” I wrote in my Nationwide Building Society diary, “and 10 minutes later they were in the washbowl upstairs!! It’s Lucozade and arrowroot for me today!” Never one to stint on the exclamation marks, I might even have helped myself to a few more had I known that, many miles away in her imposing grey slate house in Cornwall, my literary idol was at that very moment sitting down to pen me a postcard. Yes, really!!!!

“Dear Julie, How nice of you to write to me and I am so glad you enjoy my books. This is my home and appears in my novel The House on the Strand, which I think you would enjoy as much as the others. Good luck to the ‘O’ level exam in November and to the ‘A’ levels which will follow later. I’ve temporarily deserted writing novels for historical biography and it’s hard work! Yours sincerely, Daphne du Maurier.”

Four decades on, I can still recall the shiver of seeing the letter plop on to our Nottingham doormat. The neat white envelope with the exotic St Austell postmark and mysterious coat of arms printed on the back – could it be, surely not, but it had to be…? Inside, a grainy matt photograph of a breathtaking house, complete with terrace and lawns, which the author herself had bothered to label “Kilmarth” in a slightly imperfect biro scrawl. And all those words on the other side. I’d written her a fan letter and – from the impossibly far-off land of Rebecca and My Cousin Rachel – she’d actually replied. This one simple fact seemed to bring my future life into sharp focus, vindicating every single outlandish fantasy I’d ever dared allow myself.

Daphne wasn’t the first person to receive a fan letter from me. I think I was only eight or nine when, after seeing him as the Artful Dodger in Oliver! (at last, a boy I could imagine wanting to get to know), I fired off a suitably effusive letter to Jack Wild and was extremely disappointed to receive a printed form inviting me to join his fan club. It was his loss. That was the last time I ever bothered with an actor.

I had more luck, a few years later, with Sandy Wilson. Writing to him c/o the address on the back of my Samuel French play script, I informed him that we were putting on a “production” of his hit musical The Boy Friend in our dad’s garage and needed advice. His detailed reply – with a little pencil sketch of a flapper showing how the cloche hat should be worn “pulled well down” – pleased me hugely. I also took on board his suggestion that, since my little sister didn’t seem able to learn her lines, I should perhaps simply cut some.

It never once occurred to me that Daphne du Maurier might not want a Nottingham schoolgirl as a regular pen pal

But showbiz was only a hobby. I had known since the age of eight or nine that I was going to be An Author. I felt I had quite a lot in common with Shakespeare (he got off lightly: only the certainty that he was well and truly dead stopped me despatching a quick letter to Stratford). But my tastes were eclectic. A regular listener to Terry Wogan’s breakfast show, I couldn’t believe the low standard of most song lyrics and was certain that musicians all over the land needed me.

“I’ve written a good poem for a lyric and I’m going to send it to Roger Whittaker,” I wrote in my diary. “Wouldn’t it be brilliant if he used it?” And indeed, poor old Roger, c/o Westward Television, Plymouth, Devon, also figures on the “ambitions” list at the beginning of my diary for that year (though not before “Get some straight jeans” and “Start going on long walks at the weekends, to castles, etc.”)

Meanwhile another Roger – Lancelyn Green, whose retellings of Shakespeare I’d much enjoyed – seemed a good second best to the Swan of Avon. A letter from him in the Wirral tells me how “delighted” he is to hear from me “again” and apologises for his slow reply, having been away on a cruise and also that he hurt his arm. Despite this, he goes on to write two pages listing all of his other books and wishing me luck with my own writing, adding that “it took 10 years between sending my first book to a publisher and getting one published, so never despair!”

All of these letters, all of these people – some of them written to more than once (well, what was I supposed to do when the poor fools insisted on putting their home addresses at the top!) – what could I have been thinking? As far as I know, no one ever encouraged me or put the idea in my head – it just never occurred to me not to do it. It also never once occurred to me that the likes of du Maurier might not want – or need – a Nottingham schoolgirl as a pen pal. For over the next couple of years, she and I dug ourselves into a substantial correspondence. Or let’s just say, I have no record of how often or what I wrote to her (please God not too much), but I do know she replied to me at least four times.

One letter (a generous two sides of typing) apologises at length for the fact that she has no clue where I should send my poetry. Another praises my drawing of My Cousin Rachel as being “more attractive than the actress Olivia de Havilland who took the part in the film!” The last one hopes that I’ll be able to live in my dream house one day – “Menabilly was mine, but I have settled very happily here.”

‘Now, I always, without fail, answer every single letter I get from readers’: Julie Myerson. Photograph: Sarah Lee/The Guardian

I seemed to take all of this in my stride or even as my due. “Wrote to Daphne du Maurier,” I scribble in passing in my diary on 11 July 1977, but give more words to recording the tennis score and the big news that I accidentally fell asleep on the sofa. Similarly a few weeks later in August: “I must lose some weight. Wrote to Daphne du Maurier and carried on with my novel this afternoon. Granny came…”

It’s very clear, looking back through my 13, 14, 15, 16-year-old diaries (I wrote every single day without fail), that I was a strange and contradictory beast. Shy, mousy, awkward, terrified of most things (especially boys), low on physical self-esteem and largely at odds with myself (as so many teenage girls are). Indeed, all the ordinary stuff of teenage life is here: squabbles with sisters, shifting allegiances with friends at school, TV programmes watched, and a chaotic menstrual cycle which seemed capable of reducing me to quasi-invalid status.

In my head I was already someone else – someone entirely on a level with the people who received my fan mail

But I was also struggling with other, more complex things. My parents’ recent, bitter divorce meant bleak fortnightly visits to an angry, depressed father whose rage at our mother (and sometimes at me) seemed unpredictable and largely unmediated by any sense of parental responsibility. “Sometimes I feel very afraid of Daddy,” I confided to my diary.

He made sure our visits to him were uncomfortable – did not heat our bedrooms or buy much food. Sometimes he cancelled at the last moment, even when we’d been waiting apprehensively for half an hour or more, bags packed, for his car to appear. He forced details of the divorce on us – our mother’s alleged infidelity – telling us things that kids should not have to hear, even making us look through the relevant affidavits.

“I would rather never see Daddy again,” I wrote in my diary when I was just 17, “but maybe I’m too young to make such a drastic decision.”

Meanwhile, though, in the safety of my neat, bookish little bedroom, I dared to dream – and wildly so. Indeed in my head I think I was already someone else – someone entirely on a level with the people who received my fan mail, an outrageously ambitious Walter Mitty who already had a strong sense of her eventual place in the world.

In Nottingham, we knew no famous people. Nor did we know anyone who’d made a living, however modest, in the arts. The most glamorous person I knew was my English teacher – and with her painted nails, gold bangles and ability to talk as if she knew Jane Austen personally, she was awe-inducing. But both my parents had left school at 15 and my father ran a small, not especially successful, plastics factory and read nothing but engineering manuals and the News of the World. My mother, on the other hand, had read her way through the shelves of the local library all on her own and it was undoubtedly her frank passion for books which got me reading the likes of Daphne and Roger.

But when it came to life choices, she encouraged – no, demanded – pragmatism and common sense. “Mr Thomas the careers man came and he thinks I should do economics,” my diary records in tumbrel dread. “I hate economics and don’t understand what to do for the best. Mum is on one of her ‘got to earn a living’ tacks and I am totally impractical. What will my future be I wonder?”

My diaries seem to swing between a panicked desire to avoid my mother’s disapproval (and at least appear to heed the advice of the Mr Thomases) and a strong need to keep my head down and do my own thing. “Had a talk with Mummy today and made a few resolutions: I’m perhaps too creative and unrealistic and need motivation… maybe I sail along by the wind too much and ought to think about fixing a motor on!” I wrote that on 31 January 1977, but at the same time must have been writing to Daphne because another letter from her is postmarked just a week later.

‘They come from the heart’: poet laureate Sir John Betjeman’s comments on her poems make his letter the most important in Julie Myerson’s collection. Photograph: Sophia Evans/The Observer

I used to think, back then, that what I was writing was fan mail, but now I’m not so sure. Yes I certainly wrote to these people in order to show my genuine excitement for and about their work. But it’s also obvious that I sent them my own “work” – “novels” and poems and drawings (not to mention asking for their advice on my stage productions) – because I so desperately wanted to join them. (Or even, occasionally, because I thought I already had.)

Quite what it was in their work which gave me this sense of euphoric possibility rather than simple awestruck adulation, I don’t know. But what strikes me more than anything now is how deeply and valuably encouraging they all were – by being so generous with their time, their enthusiasm, their words and thoughts.

Now, in my own grown-up life as a writer, I always, without fail, answer every single letter I get from readers, for with such a history, how could I possibly not? But I do know, even with the mostly small amount of mail I receive, that it can be a slog, trying (especially after a long day of struggling to work on a book) to write more than a measly paragraph or two, trying to hit a note that will, for the reader, feel unique and properly fresh and personal, as well as friendly and appreciative. As for offering critiques on people’s unsolicited work/poems/drawings and prospective stage productions in their father’s garages – well, I simply have no idea how these poor people found the time.

That’s why the apparently hand-typed letter I received from the then poet laureate Sir John Betjeman in 1978 is possibly the most important of my collection. He tells me that he opened my letter “when I was feeling low after breakfast as I nearly always do”. He goes on to say that, though he is at the “wrong end of the production line” as far as helping me get published is concerned, he likes my poems because “They come from the heart and you are not looking over your shoulder to see what teacher thinks.”

This letter, almost more than any other, gave me such a lift. Because if the Poet Laureate Himself could tell that I wasn’t a fake, then maybe one day, possibly, just possibly, so might other people? I kept it pressed between the pages of my various diaries for many years and it now sits in a frame in my study. It still, for me, contains everything you could ever want from a reply to a fan letter: a small personal revelation, a respectfully frank response to practical queries, proof that what you sent has been read (he singles out three poems by name, oh my God, the thrill of it). And then, most vitally of all, the kind of hope that could (and did) sustain a Nottingham teenager sitting and typing away in her bedroom, for years.