When Terri was diagnosed with cancer, Lionel Shriver was doting – at first. But as her condition worsened, there always seemed to be a reason not to call...

I met Terri in the early 1980s at an arts camp in Connecticut. We were both in the metalsmithing workshop, and this sharply featured, appealingly surly Armenian taught me some new tricks. Her speciality was rivets and other "cold connections", an apt expression in her case. She was a wilful, stubborn woman, more fiercely so than I first realised; 25 years later, I'd discover just how defiant my closest girlfriend could be, even in the face of the undeniable.

Terri was full of the contradictions that always captivate me in people: inclined to bear grudges but incredibly generous (often rocking up with gifts for no reason – why, I still have half a dozen pairs of her shoes). Harsh but warm. Prone to depression but with a knack for festivity. I conjure her scowling down the pavement and rolling in laughter with equal ease. She was tortured and brooding; she was terribly kind. And she was a serious artist in the best sense: not pretentious, but determined to craft interesting work well.

Back in Queens, where we both lived in our mid-20s, we found common cause in our improbable aspirations. She wanted to become a famous artist, I a famous novelist – but Terri had then sold next to nothing and I'd not published more than my phone number. It was a big, indifferent world out there, and an ally was crucial. We'd conspire over a six-pack in my tiny one-bedroom flat, jovially certain that we'd still be best friends when we were "cancerous old bags". It was a running gag. We thought it was funny.

Beware the jokes of your heedless, immortal youth. Fast-forward through two and a half decades, during which Terri and I survived abusive boyfriends, marital problems, professional setbacks, my expatriation to the UK and her exile to New Jersey, Terri's painful endometriosis and four failed IVF treatments, as well as, of course, each other. During my regular summer migration to New York, in 2005, Terri shared her perplexity that she'd been running a low-grade fever for weeks. I said it sounded like a tenacious virus. But shortly thereafter she rang from hospital.

She was being tested for a range of ailments, the most far-fetched of these a rare disease called mesothelioma. Thus it was quite a shock when the doctors confirmed that peritoneal mesothelioma was exactly what she had – almost certainly caused by exposure to the asbestos that laced metalsmithing materials when she was in art school. Her husband Paul reported grimly that the average survival rate for this ravaging cancer was a single year.

Terri was only 50, and the timing was tragic for other reasons, too. From frustration, malaise and exactingly high standards, through most of her career she had underproduced. Yet in recent years something had loosened up, and her output had accelerated. Better still, she was at last imbuing her creations with the feeling they'd sometimes lacked, the most moving of which was an elegy to her unavailing IVF treatments. She was finally pulling in big commissions, one of which was about to go on display at the V&A. At the same time, her brooding demeanour had brightened; she'd grown more outgoing, energetic and relaxed. Almost... happy. Well, so much for that.

On the heels of her diagnosis, I was doting. I'm not tooting my own horn. I suspect being a paragon at the very start of a loved one's illness is pretty much the form. We're on the phone daily. We stop by regularly, and bring freshly baked scones. We follow every medical twist and turn. And we're inclined to rash promises. With a flinch, I recall declaring before Terri's surgery that I'd be willing to move into their house in New Jersey for weeks at a time! I'd be at her beck and call, running errands, preparing meals and filling prescriptions.

Useful tip: if someone close to you falls gravely ill, at the outset, in the first flush of anguish and desperation to help? Watch the mouth.

For the timing of Terri's cancer was terrible for me as well. A month after her diagnosis, I was intending to return home to London, where a host of professional commitments could not (or so it seemed) be reneged upon. Although for most of my literary career I'd scribbled in obscurity, my prospects were suddenly looking up. My seventh novel had inexplicably hit the bestseller list in the UK, and subsequently won the Orange prize earlier that summer. (I still have the droll good-luck package Terri and Paul delivered when I made the shortlist: orange marmalade, orange candles, orange oil.) For the first time, I faced a smorgasbord of opportunities – festival gigs, bookstore appearances, feature assignments – and I was in the middle of a new book.

So, however reluctantly, I flew back to London. After Terri's surgery, Paul phoned with the lowdown: the surgeons had discovered a patch of aggressive "sarcomatoid" cells, which meant Terri's prognosis was bleak.

I will give myself this grudging credit: I did fly back to visit Terri for Thanksgiving that November, and for a while I kept in faithful touch, ringing weekly and following every grisly detail of her punishing chemotherapy. But this is not a boast about what a wonderful friend I was in Terri's time of need. This is a mea culpa.

Little by little, I'd notice that it had been a fortnight since I'd rung New Jersey. I'd kick myself. But some book review would be due that afternoon, so I'd vow to ring tomorrow. Time and again some immediate task would seem more urgent, and I'd tell myself that I should ring Terri when I'm settled and concentrated. Watch out whenever you "tell yourself" anything; it's the red flag of self-deceit. Long hours of being "settled and concentrated" mysteriously failed to manifest themselves.

I stuck a Post-it note on the edge of my desk: "RING TERRI!" Over the months, the note faded, much like my resolve. On the too-rare occasions I acted on the reminder, I had to put a mental gun to my head. But why? This was one of my closest friends, and she was dying. While she was still on this Earth, why was I not battling to maximise every moment? Surely the problem should have been my ringing too often, whizzing back to the States too many times, making a pest of myself.

Granted, our conversations were sometimes awkward. My own life had never gone more swimmingly, while Terri's was circling the drain. I was embarrassed. I found myself editing from our discussions anything I'd done that was exciting or fun. When I returned from an author's tour of Sweden, I portrayed the trip as a drag. This  sort of cover-up reliably backfired. So apparently I felt sorry for myself – for going to Sweden! When Terri could rarely leave the house.

I make no apologies for this, since this is what novelists do: at some midpoint in Terri's decline, I decided that my next novel would draw on this encounter with cancer. At least I had the humanity to refrain from taking notes during our phone calls, thus relinquishing many a "telling detail" and much "great material". Consequently, I had to do an enormous amount of research on mesothelioma later, and this is what I do apologise for: not having done all those web searches on her treatments – the surgery, the drugs, the side-effects – when Terri was still suffering through them. Now, I'm mortified to have Googled "mesothelioma" only once the search was for a book.

When I returned to the US that second summer, Terri had alarmingly deteriorated. Thin to start with, she'd lost weight. She was gaunt and weak, her skin tinged a dark, unsettling orange: a chemo tan. It was obvious where this was headed. But whenever anyone acted as if she wasn't going to make it, Terri grew enraged. She resented the "sentimental" testimonials her friends and relatives recited at her bedside; she thought they were delivering a death sentence. Though she wouldn't have put it that way. I wonder if throughout her illness I ever heard her say the word "death" aloud.

Thus on one count only could I blame Terri herself for my increasingly deficient friendship. Her refusal to admit she was dying meant we couldn't ever talk about the elephant in the room. Pretending that the treatments were working and she was going to come through this injected an artifice in our relationship at odds with the confidences we'd shared for 25 years. Days I did visit, afternoons I did ring, we'd end up talking, lamely, about recipes. Indeed, on a brief trip in November 2006, I visited Terri in New Jersey; it was the last time I'd ever see her, and I knew this instinctively at the time. Yet we spent an appalling proportion of that final visit talking about mashed potatoes.

When her husband rang me in London a few days later with the news, he was consumed with a steely rage. Obviously Paul was angry that he'd lost his wife. But he was also angry at other people. Oh, he expressed his disgust in general terms, as a disillusionment with the human race, a good-riddance to our whole species. But I knew what he meant. Paul's fury was aimed at Terri's friends and family, who had almost universally made themselves scarce for months. His fury was also aimed at me.

I thought I deserved it. I had visited, some. I had rung up, some. But not nearly often enough, and in truth one of my best friends perishing before my eyes had instilled a deep aversion, an instinctive avoidance, a desperation to flee.

It would be a far better thing if I were a lone shithead amid an ocean of altruists. And surely some folks really do step up to the plate when a friend or relative falls mortally ill – wonderful people who keep popping by with casseroles to the very last day. I have a new admiration for such stalwarts, as well as a new appreciation for the Christian duty to "visit the sick". Yet I fear this suddenly-remembering-somewhere-you-gotta-be is a common failing of our time. In fearing and avoiding death, we fear and avoid the dying.

I'll risk sounding preachy, since I've paid for my sermon with a regret that never leaves me. Most of us will experience the afflictions of our nearest and dearest perhaps multiple times before we're faced with a deadly diagnosis of our own. So be mindful. Disease is frightening. It's unpleasant. It reminds us of everything we try not to think about on our own accounts. A biological instinct to steer clear of contagion can kick in even with diseases like cancer that we understand rationally aren't communicable. So the urge to avoid sick people runs very deep. Notice it. Then overcome it. There will always be something you'd rather do than confront the agony, anxiety and exile of serious illness, and these alternative endeavours seem terribly pressing in the moment: replacing the printer cartridge, catching up on urgent work-related email. But nothing is more pressing than someone you love who's suffering, and whose continuing existence you can no longer take for granted. So never vow to ring "tomorrow" – pick up the bloody phone.

· So Much For That, by Lionel Shriver, is published by HarperCollins on 25 March at £15. To order a copy for £14, with free UK mainland p&p, go to guardian.co.uk/bookshop or call 0330 333 6846.

• Bereavement
• Relationships
• Cancer
• Health