On the last day of 2016, I found myself stomping around my mother’s kitchen, convinced that my husband of 21 years was going to leave me for a Kenyan climbing guide.

‘He does love those mountains,’ Mum agreed. ‘But he’s a decent man. He might only leave you for a short while.’

A thought struck her. ‘Isn’t it like that book you’ve just written? It’s nearly like you made this happen.’

True. I was finishing a novel, The Break, about a man who asks his wife for six months off ‘for good behaviour’ to do all the things he’d missed out on because he was married. Was life imitating art?

I love my husband a huge amount. Even so, I feel that a romantic relationship is just like any other: one with a sibling, a long-term friend or a colleague (except for the sex bit, and we’ll get to that).

Marian Keyes (pictured right on her wedding day) shared the unexpected sabbatical from her marriage after 21 years together

Sometimes you’ll be in perfect harmony, loving the same stuff, never running out of things to talk about, laughing like drains; and then — suddenly — the sound of them eating an apple will make you want to bury an axe in their head.

The younger me would be appalled by my pragmatism. I spent my 20s looking to be completed, hoping for a man to appear from nowhere and Polyfilla the holes in my soul.

I thought love was jealous misunderstandings, shouty departures, slammed doors, dramatic reunions and finally, a perfect life in which I’d never again face loneliness, insecurity, infidelity, abandonment, fear or boredom.

A glittering lacquer called Love would deliver a steady feed of those similar-to-cocaine chemicals that mark the start of a relationship. Love would take care of itself, requiring no input from me or Polyfilla Man.

When I was 30, I met The One. The Best Man in the World. (Saying this fills me with a terror of being struck down by the god of smugness.) There were far fewer of the histrionics I thought were compulsory.

We were friends first. He was quiet and kind — qualities that weren’t on the list for my ideal man, which included such delights as: ‘Artistic. Tormented. Prone to sudden, destructive rages. Liable to disappear for four days without explanation.’

At the wedding, a couple of well-meaning old-timers had the temerity (as I saw it) to issue solemn warnings that marriage ‘takes work’. I was polite, but secretly scornful — clearly they had married the wrong person, unlike me, who had taken the precaution of marrying Mr Perfect.

Marian was warned that becoming the household breadwinner could emasculate her husband and cause problems within their marriage

In addition, I had no idea what this ‘work’ entailed. I had vague notions about never going to bed on an argument, keeping the magic alive, blah-dee-blah. The old-timers should have given specifics. But not to worry. All in the fullness of time.

When we got married, my husband was the one with the good job. Less than a year later, I got a book deal and became our household’s breadwinner. Himself gave up his job to work as my assistant, a temporary arrangement that became permanent and, once again, people ponied up unwelcome advice.

According to some naysayers, the implosion of my marriage was inevitable because we spent too much time together. Others warned that by bringing home the bacon, I was emasculating my husband and it was only a matter of time before he remasculated himself by having an affair, which would have been catastrophic because I have a zero-tolerance policy on infidelity.

As far back as my teenage years, my girlfriends and I were super-sneery about famous women who forgave their cheating partners, decreeing they had no self-respect. We were adamant we’d scratch the cheater’s car, microwave his vinyl and never, ever forgive.

She says they began hurtling towards mid-life crisis as she inched into her 40s

Despite everyone telling me I was doing marriage wrong, my husband (to the best of my knowledge) didn’t have an affair. Neither (to the best of my knowledge) did I. We didn’t implode. In fact, we were happy. He was my favourite person by miles, but I kept that to myself because it’s considered bad form to say you love your person.

Despite that, we hit bumps in the road. We wanted children and couldn’t have them. My shiny new career generated friction. I worked like a dog, saying yes to everything, afraid that if I didn’t, I wouldn’t be asked again.

Conflict arose because Himself wanted me to do less and I couldn’t slow down. As well as the external stuff, it came as an unpleasant surprise that my old internal demons were as lively as ever. (They’re nothing special: fear of failure; small, sharp shards of shame; dread of abandonment; horror that that b**** from school is more successful than me.)

I had genuinely expected the love of a good man to magic them away. But as the years passed, I understood that no one, no matter how much they love us, can erase the uncomfortable emotions that go with being human. It’s not their job.

Inching into my 40s, I still loved Himself very much, but from watching the relationships around me, it was clear that even the best connections can unravel. Everything is lose-able, and nothing is guaranteed. Because people change. We’re all changing, sometimes quickly and sometimes all at once.

If you’re lucky, you both change in roughly the same direction at the same speed. Meanwhile, both Himself and I were hurtling towards mid-life crisis years and I no longer thought our special union had immunity.

There are forces more powerful than love, such as our wretched lizard brain. It goes along keeping a low profile, then notices we’ve lived more than half of our expected lifespan, realises time is finite and says: ‘Hold on, I’ve got living to do!’ This ‘living’ might mean buying an impractical car or running a marathon.

What if Himself did run off, but soon changed his mind and came back? Could I live with that?

But we’ve been conditioned to expect that most of our happiness will come from a relationship, so dissatisfaction tends to be blamed on that. A new person will make us happy again, right?

To stave off an attack by a mid-life crisis, I’d now and again ‘make an effort’. This involved asking Himself questions and behaving like I was listening to his answers. I also issued instructions: ‘You’re not to have a mid-life crisis.’

But the person who had the mid-life crisis was me. Well, I had a crisis and I was 46. But instead of wanting to live more fully, I wanted to die. I spent a couple of grim years in the wasteland of depression. Meanwhile, Himself had his own mid-life crisis: he started climbing mountains.

I hate any kind of incline, so his mountaineering is done without me. But I worried about him meeting a new lady friend on a snow-covered peak, a person who shared his passion. So whenever he went off, I inquired, in a high-pitched tone, if there were any women in the group.

The thing is, he’s a decent, loyal person and we’re very close. He insists he can be trusted and I believe he means it — but I also believe most people are strangers to themselves. Stacked inside us are millions of experiences and longings, most of which never surface. But every so often, something gets triggered that propels us on to a different life path.

Marian was comforted by her mother that her husband would stay faithful whilst away climbing in Kenya for 12 days

A few years ago, I read about a new kind of mid-life crisis which has developed because we’re all living longer. People affected by this love their partner and ultimately want to stay with them, but if they’re looking down the barrel of another half-century before they peg it, they’re starting to want ‘a break’.

Unlike silver-splitters, these people want a sabbatical, not a divorce. Some involve something sexually benign, such as travelling. Other people want a time-limited period to behave as if they’re single again.

This is a different spin on the traditional mid-life crisis, which features covert infidelity; it’s above-board, sanctioned cheating. ‘Conscious’, if you will.

I thought: ‘Oh great! A new thing to worry about!’ So, oddball that I am, I decided to write about it.

Every relationship is a mystery, revealed only to those who are living it; unless it’s abusive, it can have whatever setup they choose

The Break is about Hugh and Amy, a couple in their mid-40s who have been together for 18 years. Hugh’s father dies and he suddenly knows, on a cellular level, that one day he will die, too. The seconds and hours of his life are pouring like sand through his hands and he’s panicking that he’ll never again feel the freedom of his youth.

After soul-searching, he tells Amy he wants to be temporarily single. He’ll get that corrosive yearning out of his system, then he’ll come back and be fine.

To write the book, I had to put myself in Amy’s shoes and experience her fury, humiliation and, most painful of all, her sorrow. I found it difficult because I love Himself an awful lot.

Last September, my father-in-law died, then in October, a close friend of ours died. In December, as I was finishing the book, my husband went on a 12-day climb of Mount Kenya. He’d be out of coverage for most of that time.

When he’d met the rest of his group, he called from a crackly satellite phone to say his goodbyes. I asked how many people were on the climb. ‘Two,’ he replied. ‘Plus the guide.’

Quickly, I asked if there were any women. A pause. A crackle. ‘Yes,’ he said. ‘The guide.’

Bizarrely, my next question was: ‘What nationality is she?’ (In my head I carry a Sexy Nations League.) ‘Kenyan,’ he said. Then the line went dead.

The Break by Marian Keyes (Michael Joseph £20) is out now.

Instantly I surrendered to my worst fears. I could see it all: him and this fit, fearless Kenyan woman roped together at high altitude, utterly dependant on one another for their very lives.

My mother provided some wisdom: ‘Maybe the other man will get off with the girl.’

But my imagination had given him altitude sickness, so he’d had to abandon the climb.

‘Himself is very trustworthy,’ Mum said. ‘You couldn’t meet a better man.’

‘But his dad has died, he’s come face-to-face with his mortality and there’s every chance he’ll never come back. He’ll set up a small company in Nairobi,’ I said. ‘Him and this woman. Offering guided mountain climbs to adrenaline junkies.’

Mum looked stricken. ‘Living his best life . . . Like your book.’

The rational me knew I was being ridiculous, but the part of me that’s skilled at catastrophising had the upper hand.

During the days of radio silence, I contemplated my attitude to fidelity. What if Himself did run off, but soon changed his mind and came back? Could I live with that?

I didn’t know, but the zero-tolerance attitude of my younger years no longer felt relevant. Rigid templates don’t work because life is lived in nuance.

And I realised something else: as a society, we’re far too quick to pass judgment on the decisions of others. Every relationship is a mystery, revealed only to those who are living it; unless it’s abusive, it can have whatever setup they choose.

As it happened, Himself came back from Kenya — and revealed that the ‘other man’ had also been a woman.

The Break by Marian Keyes (Michael Joseph £20) is out now.