I should have seen it coming. We'd been together for over a decade but for the past year or so things just hadn't been the same. There was no big betrayal, no shocking infidelity, no bitter rows about money, but something had started to fade, the spark had gone. Social events became awkward. We couldn't even have a cup of tea at our favourite cafe without there being an atmosphere. Before long, what had started out as drifting apart turned into irreconcilable differences. The end, when it came, was civil yet brutal: "We may as well call it a day," I was told.

In shock, I called a friend and relayed the conversation I'd just had. "Oh my God, you've been dumped," she said, confirming what I already knew: I had indeed been dumped but not by a long-term partner or spouse; the dumper was a friend, or to put it more accurately, a former friend.

When a sexual relationship ends, things are relatively simple. There is the dumper, and the dumpee. You have the, "It's not you, it's me" conversation. There are broken hearts, recriminations. The relationship is declared officially dead. You decide who gets custody of the children/cat/Wire box set. Mutual friends choose sides. No matter how devastating, at least it's clear: you were a couple, you exchanged bodily fluids. You are no longer a couple, you keep your fluids to yourself, or distribute them elsewhere.

But when a friendship cools, it's seldom so straightforward. Death and betrayal aside (sleeping with someone's partner is a pretty swift way to sever ties), there are two ways to end it. There's the slow fade-out, where you try to downgrade an intense friendship to something more casual. Or the short, sharp split. "I sometimes felt I was the perfect customer for a much-needed but never-produced Hallmark card that would read: 'We've been friends for a very long time now,' followed on the inside by, 'What do you say we stop?'" writes Joseph Epstein, author of Friendship: An Exposé. But if such a card existed, how many of us would have the guts to send it? And if you received such a note, would you think, "Fair enough" or would you want to send it back with a few added expletives?

The crucial thing with the slow fade, of course, is whether or not you choose to inform your friend of their demotion or just avoid them until they take the hint. (By the way, the slow fade-out is not to be confused with "secondary" – or more casual – friends. Secondary friendships are on an equal footing, rather than one person wishing they didn't have to see so much of the other.)

Unsurprisingly, I am not blameless in my own friend-dumping debacle. Six months ago I radically changed my lifestyle by going freelance in order to write my second book. I went from having the same routine for 14 years, to the unpredictability of a "portfolio" career. Now there was no one paying for my down days, my time became more precious. If I wasn't writing, I wanted to hang out with writers, creatives and the kind of people who consider 8am a perfectly reasonable time for a Pilates class (no need to rush to the office for a 9am start). Also, a couple of family dramas have taken any spare energy I had left. Under this blustery climate, certain friendships have thrived and others have suffered.

With me and my former friend, differences in taste and ambition crept up on us. Whether it was local politics, the financial crisis or private education, every issue seemed to highlight how different we were. We met one afternoon when, for some reason, I was raging about Alexa Chung: "I hate those Mulberry Alexa bags. Why would anyone want a bag named after a slightly rubbish TV presenter?" I said. Out of the corner of my eye I saw she had a new bag. "That's an Alexa, isn't it?" I said. "Yes," she said.

As we struggled to find common ground, it became tricky to talk about anything that mattered. My friend tried to reconnect with me a few times; meanwhile, I called her less. Very brave, I know. Eventually, she must have had enough of me fobbing her off. In our last conversation it became clear we disagreed on one fundamental principle: I was happy to let things drift, she was not. Who was right?

"Sometimes it's possible to downgrade the relationship by seeing the person less or to dilute it by seeing the person with a group," says Irene Levine, professor of psychiatry at New York University School of Medicine and author of Best Friends Forever: Surviving A Breakup With Your Best Friend. "Sometimes, you just need a break from the other person. I call it a 'friendship sabbatical'." A friendship sabbatical? It sounds so civilised. Presumably you either return to the friendship fully recharged or decide to retrain as a yoga teacher. Mine was more like a friendship divorce.

You can see the slow fade-out as the coward's way or the kinder, more respectful way of letting down someone you once cared for and who probably still cares for you. If you have a lot of mutual friends, or are likely still to see each other, "downgrading" makes things less awkward, but if the friend is too naive or self-absorbed to read the signals, or just really persistent, it may not be enough. Eventually you may need to do the decent thing and dump them properly.

And that takes courage and honesty. It doesn't have to be cruel but it does mean telling someone you were once close to why you feel they are no longer worthy of your time. No one likes to hear that they are surplus to requirements, so tread carefully, warns Jodyne L Speyer, author of Dump 'Em: How To Break Up With Anyone From Your Best Friend To Your Hairdresser.

Speyer suggests a "pre-dump": "Give me a warning, maybe I can fix it, and if I can't, then at least I knew this was coming, so it prepares me. And be kind about it, say, 'Here's what's not working.' I don't need 100 reasons, don't give me a whole laundry list, but let me know what the problem is, so I can have that information and move on. I don't have to agree with it, but at least I have something."

Speyer wrote the book because she was having so much trouble terminating unwanted friendships but says it gets easier with practice: "When you're clear about your feelings, other people respond to that. You may think you're dong someone a favour by not telling them but in the long run it's a lot worse."

She then tells me a rather terrifying story of how she got rid of a childhood friend who moved back into her neighbourhood. "She crept back in after a 10-year gap. In her mind we were best friends again, she was calling me every day. I was psyched to have her back in my life but that wasn't what I wanted. So I said, 'Let's have lunch every three months instead.'"

Still, however gently you do this, chances are someone will feel hurt and resist. "Most friendships, even very good ones, don't last for ever," Levine says. "Yet women are raised to believe the romanticised notion of Best Friends Forever. In our culture, women are judged by their ability to make and keep friends, so women have a hard time getting over the loss of a best friend and see it as a personal failure."

Some sexual relationships, she argues, are easier to let go of than a truly intimate friendship. Things are made worse by the fact that many end so slowly, withering and dying after months or years of neglect with one or both sides building up resentments. When my ex-friend (quite reasonably) asked me to return some books my partner had borrowed, I took offence. When she refused the offer of a drink at a party, I felt snubbed. Perhaps if I'd been more honest, our relationship wouldn't have foundered. I tried a few times to say that I felt we didn't have as much in common any more but I also wimped out, blaming our paths not crossing. If I'd really wanted to see her I'd have found a way; instead I felt guilty every time she left me a voicemail.

"When friendships drift, we rarely discuss it," says relationship psychotherapist Paula Hall. "If it feels as if one person has made less effort, then you can feel rejected. Because friendships don't end in a conflicted way, there isn't closure, you don't feel you're better off without each other, it just stops, so there can be feelings of loss."

My own dumping is still recent enough to feel raw. I'm not sure which hurts my pride the most, the fact that I've been dumped, or the knowledge that I have failed as a friend. I worry what mutual friends think and how uncomfortable it will be when we inevitably bump into each other. I worked myself into a rage when she clicked "like" on my sister's Facebook page. How dare she, I spat, that's my sister.

No matter how mature we may be in other areas, friendships return us to the school playground quicker than you can say kiss chase. Sites such as Facebook and Twitter simply highlight this. "You don't go up to someone on the street and say, 'Do you want to be my friend? Yes, no, poke me.' But on Facebook it's much more in your face," says psychologist Linda Papadopoulos.

Much has been written about friends being the new family but new technology has also changed the nature of modern friendship, creating weird new hierarchies. A Twitter friend is lesser than a Facebook friend, a Facebook friend is not someone you'd Skype.

"What one wants from – and is willing to give to – friends feels different than it once did," says Epstein, who likens today's friendships to the seating in a sports stadium: your closest friends sit with you in the box seats, secondary friends are in the grandstand seats, "and the rest – business acquaintances and associates – are in the bleachers (the stands). These days, there's another category – people with whom you have only a virtual friendship, they're up in the nosebleed seats."

But there is hope, even in the cheap seats, because friendships aren't static, so people can move from one area of the stadium to another. Someone who starts out in the stands – perhaps a work colleague or neighbour – can be promoted via the grandstand to the box seats.

But the reverse is also true. If I have any friends left after this article, I'd like them to know that I'm not avoiding their calls, I'm just really busy.