From across the office, I watch a man, a colleague, talking to three others in my department. I take in every detail of the Man: his soft blue eyes, his strong freckled arms and, when he glances briefly over to me, a smile which seems so warm that I have to clutch my chair to stop myself from running over to him.

I’m falling in love. With every day that passes, I find there is less room in my heart for any other person.

But it’s not that simple: I’m with someone else. I own a flat with Ben, my partner of four years, and my friend of eight. And what’s becoming clear to me is that although I love him, I am not in love with him.

I fight my feelings. Ben and I share mutual friends. We have a gorgeous home. There’s a trip to Berlin that’s been planned for my 30th birthday, and friends are whispering that Ben might propose. It feels like that’s what I should be doing – settling down. But everything that previously felt entwined now feels entangled.

I imagine how I’d feel if Ben did ask me to marry him. A ring? Lovely. A party? Even better. But when I think about what being married means, I don’t feel excited at the thought of doing it with him. I feel a bit flat. At work I’m animated, happy to be in the company of the Man. He’s witty, he challenges me; he brings me to life. At home, I don’t like the person I’m becoming – I’m demanding, bitchy. Ben is worried and withdrawn.

I can’t tell anyone how I feel about the Man – they all like Ben. And the Man has been married before; he has a four-year-old daughter. Baggage, they’d think, but to me, hearing him talk about her just makes me want him more. Ben and I go to Berlin. All I do is miss the Man. Ben doesn’t ask me to marry him.

Then, on a work night out, the Man and I kiss. I feel guilty, but not that guilty. I lie awake at night staring at the ceiling, as Ben sleeps next to me. I don’t feel like I can go back, but I’m too scared to go forward.

Can I walk away from Ben, my life, my home, my friends – for what? A kiss? I eat less, I drink more, I get angry with Ben. This is his fault. Why can’t he be more like the Man, and less like, well, him? But Ben is kind and gentle, and I feel guilty for blaming him.

The Man and I meet. But I don’t want to have an affair. I want what I now know he wants too – to be together. I can’t allow myself to imagine it yet. But then I do, and the most intense sensation of relief and happiness washes over me. I can do this. I just have to take a leap of faith.

Because I have a feeling, deep inside me that I can’t shake off – that this is the love of my life.

I tell the Man. He feels the same. It’s a strange start to a relationship – we have to be serious from day one. I’m not going to walk away from everything for a bit of casual dating. But then I realise I’m not walking away from everything, I am walking towards it.

I tell Ben. It’s horrible. I am hurting someone who hasn’t hurt me, who hasn’t done anything wrong, but I know I’m no good for Ben. I know what being in love feels like now, and I know he’ll feel that too, but with someone else, not me.

That was then. This is now, 12 years later. I married the Man, Andy, a couple of years after I left Ben. And I was right – he was, is, the love of my life. We laugh, we fight, we are lovers and we are friends. I still want to run towards that smile.

We have two children. And when they ask me how their mummy and daddy met, I tell them we worked together in an office. And that although I tried not to fall in love, I couldn’t help it. They ask why I tried not to; I tell them I thought life had a different plan in mind for me.

But it turns out that life doesn’t make plans for you – you make them for yourself.

Photograph: iStock