For an intelligent, confident and not unattractive 35-year-old, I have a disastrous relationship history. I've dived into passionate long-distance flings, forced friendships into more, with terrible results, and rebuffed perfectly lovely men for no reason at all. I never wanted to be on my own, but spent my 20s more single than not.

My last relationship – at two-and-a-half years, the longest and most important of my life so far – ended messily about 15 months ago. I felt battered, exhausted and desperately sad. I broke off all contact with my ex, left the street on which we lived and curled up in my friend's apartment for the summer.

Had I known how transformative this break-up would prove to be, I would have felt a lot less troubled. But those weeks marked the start of my recovery: my sunny side – submerged during my relationship – began to pull my other self, the one who wanted to sob every night, out of her hole.

A year, a fling, a backpacking trip and a lot of self-discovery later, I am happier than I've ever been. It shows in my face, my work, my social life, even my health. But I'm single. I'm not alone: the number of single households is rising and predicted to reach 40% this year. A recent survey reveals seven out of 10 Londoners have dated in the past year – underlining how few of us are in long-term relationships. We have too much choice, the experts say, which counterintuitively makes it harder to meet people. So we're single – and stuck that way.

For me, until recently, this was a choice: I was recovering from the end of a serious relationship. But in the past months, my lack of boyfriend has loomed larger. So I've agreed to talk to Andrew G Marshall, a therapist who specialises in relationship and singles' counselling. Marshall, author of The Single Trap, has been helping single people since he started counselling 25 years ago, but in the past five years has noticed an increase in their numbers, and in their despair. "Most prefer to plunge into the next date, rather than understand why they keep meeting the wrong people, or find it hard to meet the right ones," he says. "And online dating has made things worse. The biggest problem is not enough knowledge about relationships, or skills to handle them: compromise, the ability to argue constructively, trust."

He sees people, on average, for around eight weeks – and sees four times as many women as men. "The men I see are normally trying to come to terms with a failed relationship – they tend to be older, and totally at sea because they don't understand what went wrong and have no one to talk to. Women are more likely to be looking for love, and come into counselling as a last resort – often after what they felt was their last hope relationship ended. Women are socialised to be interested in relationships, which makes them better equipped for them, but they feel more of a failure if they are not a success." Older people are more likely to approach him, he says. "It's easier to settle in your 20s as you're more flexible. In their 30s, people think: 'I've put up with so many losers – this [relationship] had better be brilliant,' and that's not a good attitude."

I'm not sure which category of single I fall into. I'm intrigued to see what Marshall makes of me and my dating record, but if I'm honest, I want to learn something useful that will help me find a good boyfriend.

After asking my age, he gets straight to the subject that is, surprisingly, at the core of his thinking: my family. Are my parents still together? Describe their relationship. What is it like having a twin brother? Do we get along? "What we learn from our parents about relationships informs the way we think about them," Marshall says.

He then asks me to name my significant boyfriends, and writes them down in a tree-shaped timeline. It's a bit unsettling seeing them gathered there.

When he's finished, we peer at the diagram. "So, between 20 and 35, you've probably not been in a relationship for more time than you have," he says delicately. "Have you any thoughts as to why that might be?" Sure, I say: insecurity, lack of confidence, inexperience. "But I have a picture here of you being quite a confident girl," he says. I suddenly feel emotion engulf me. "Do you want to tell me about that?" he asks.

So we head back to my school days: me, tall and skinny with braces on my teeth, hating my looks and harbouring secret crushes – a pattern that would continue throughout my 20s. "So you didn't fear boys because you thought they would take you over and run your life?" Marshall asks. Quite the opposite, I say: I wanted to go out with them, I just didn't know how to.

We then talk boyfriends: what were they like? How did I meet them? How was the sex? What sort of person do I think I'm suited to? He laughs along with me, nodding, smiling, scribbling and asking pertinent questions.

Then we stop. I sip on my water while Marshall looks thoughtful. "What do you think when you look at your family, and those people you've been out with?" he asks. "Does anything spring to mind?" And for the first time in our session, I'm silenced. A blank.

He continues: "I have hopefully made you think about your family and your relationships. I just wonder if, when you look at them, you see anything that strikes you." I stare at the chart: I hadn't realised how many boyfriends had three-letter names. But I don't think this is the sort of insight Marshall is after, so instead, I mutter something about it all being a bit of a disaster. "Disaster?" he says. "No, I wouldn't say it looks like a disaster by any stretch of the imagination."

But my inability to draw conclusions from my own history doesn't matter – Marshall has his own, and they are so unexpected I can't believe I never made the connection before. It is, it seems, all about my brother.

"It's considered quite a hard legacy to be an opposite sex twin," he says. "On one level, you're incredibly relaxed with boys: they're terribly familiar, because you've been with a boy your whole life. But being different from them is really rather difficult. Sexually, they're incredibly ... well, where do you start?"

I find this wonderfully comforting, as if someone has just thrown a warm blanket around my shoulders. It all makes sense: the fact I find men so easy to get on with, the fact I find flirting so uncomfortable. There's more. My parents – same age, background, career, together since school, happily married – compound this feeling of familiarity. "For attraction, we need difference, otherwise there's no grit, no passion," says Marshall. This is why the men I'm attracted to are so different from me, he says, and why the ones just like me, from similar backgrounds, don't work out.

There is such a thing as too much difference, however, and that's why my last relationship foundered: we wanted different things – I, commitment, he, freedom – and those are insurmountable challenges, says Marshall. "You musn't go back to that level of complexity, but you have to look for difference, allow it and not get panicked by it. What you don't need is nice and sensible, because there's no passion there." I feel an enormous sense of relief.

What about my legacy, I ask. Can I change that? "You can't be something you're not," he says. "You're honest, and I think men might find it appealing to have a woman who's not trying to be deeply mysterious. But men are frightened of rejection too – they only pounce if you are a dead certainty, so you need to let them know."

Our session ends. I feel light‑headed and giggly, as if someone has just made sense of me. Am I typical of most clients, I want to know. "I was expecting something more complicated: divorced parents, multiple relationships that have left you bruised," he says. "Most people who haven't found love over-date." Can you under‑date? "I didn't think you could until I met you. But you have a sunny outlook: 18 months is a good time to get over someone important. I don't think there's much to be worried about."

I wish I'd seen a therapist like Marshall in my 20s. Back then, I thought therapy was for over-analytical New Yorkers in Woody Allen films, not normal people. No one else I knew had trouble meeting men, so I would have felt embarrassed having counselling. Does this attitude still exist?

"There is still a taboo about asking for help," says Marshall. "People wait about five years too long. But therapists have been slow to publicise what a profound difference we can make to single people, so most people don't even think of asking for our help – which is a pity."

I don't believe we should blame therapists: in my experience, we are slow to talk to professionals about our love lives because we have no precedent – from school onwards. But I also believe a cultural shift is taking place: therapy is openly discussed and depicted in the media, and as a nation we are, good or bad, becoming more emotional.

In the days following my session, I turn everything over, ex-boyfriends invading my dreams. A week later, I have two encounters. First, I meet an old friend for a drink and look at him in a new light. Different from me, he's someone I would never have considered boyfriend material – but now? Then I meet an attractive guy at a party and – inwardly cringing – flirt with him. It is surprisingly fun and I leave with a skip in my stride. To an outsider, these are small steps, but for me, they're giant leaps into, I hope, a happier relationship future.

The Single Trap by Andrew G Marshall is published by Bloomsbury