In the middle of supper with friends on the evening I later found out about the affair, my beloved husband got up to go to the loo. I remember thinking that he was taking rather a long time, but he came back smiling and I thought no more of it.

We arrived home after midnight enthusing about a few of the people we had met. We'd had a good time. I was knackered and went up to bed. He went to his study to wind down in front of the computer, tweaking at his work, with a glass of wine. I think this is quite normal - certainly I have a lot of married friends who prepare to go to sleep in different ways after a night out. Sensible, not sinister.

It took me some time to get ready for bed, all that tedious flossing and removing of mascara. When I finally made it, I considered for a moment whether or not I had the energy to read. As I did so, I became aware of a muffled voice coming from my husband's study.

On the phone, I thought. Funny, it's quite late. Must be talking to Jack, his best friend. But then again, although I couldn't make out the words, there was something about the tone. Not quite the way he would usually talk to Jack. I tensed without knowing why.

My husband joined the Mile High Club

Our anonymous writer plots her relationship's course - and her husband's affair - in the first week of diary of a marriage

10 things to know before confessing to an affair

If you've had sex with someone other than your partner and are thinking of spilling the beans, read this first

How to own up to an affair

Our relationship expert says although an affair can feel right, you may have suspended a lot of guilty feelings to keep it going

BACKGROUND

• My lover has a girlfriend and kids

• Should I leave my husband for my lover?

• Should I end my affair for cheating fiance?

Oh, God, just thinking about it now, by the glum, Wednesday morning light of my computer, even though it was several months back, gives me pain. The throat constricts very slightly and when I swallow it is as if a small gag of coal is loosely stuck there. Onion eyes.

Something tugged me out of bed. Who's to say what? Instinct? Something made me creep so he couldn't hear my tread. I had no picture in my mind, no thoughts. The body was working of its own accord. Brain only aware that something was not quite right. Who's to say what?

I crept downstairs knowing only one thing, that I mustn't be heard. With each tiny murmur of the stairs, I stopped in terror before daring to inch forward again. So painfully slow, my progress. I put my ear to his closed door. Silence, then one or two whispered words. Affectionate sounding. Grown-up-secrets sounding. Again, couldn't make them out but knew they were not to Jack. Of course it was a woman, and I guess I knew it then, at that moment, but the thing that fascinates me was the body and the mind, and the relationship between the two at that vivid second of realisation, that millimoment of movement between Not the Faintest Inkling and He's Having an Affair.

The mind had two thoughts: first, world as I know it, falling apart. Second, I don't give a s**t; what does it matter, pig, bastard, s**t, f*** off with her then, don't care, hate you, hate her, good f***ing riddance, HAVE each other then, stew and suffer for ever, f***ers.

As for the body. Well, all the usual suspects - I held my hands close to my face and, sure enough, they were shaking. My stomach - the clichés always have it down as the stomach, but in fact it was my intestines and whole arterial system as well - felt as though it had turned to liquid metal. I did not collapse to the floor like a Victorian lady but I could feel the bowels shifting queasily. I can't remember if I felt sick, I suppose I did, but I can remember there was a small piece of me that was able to watch the body with dispassionate amazement at its instantaneous and total response.

Through the pain, there was a small, very small part of my mind that also thought, well whatever's going on behind that door, the outcome ain't going to be boring at any rate. And, I am b*****ed if I am not going to rise to this challenge.

Evil nugget of me steeling myself for the fight. Part of me arrogant enough, through the terror and confidence crash, to know that, whoever she was, she wasn't up against a pushover, and she was not going to get the man I still love (and have done for almost as long as I can remember) without a struggle, indeed if at all. Ultimately.

I crept upstairs again, stood in a stupor at the top, then went back down. Ear to the door. More non-Jack-tone whispered words. Up again. Down again, third time. Silence behind the door.

I hadn't wanted to interrupt their conversation (same courtesy? dignity? that prevents me reading his texts), but it seemed as if it was over. So I flung open the study door. Stood there, ugly in my frumpy towelling dressing-gown. Calm voice shot through with lead:

“Right, so who is she?”

Next week: I wanted to know absolutely everything, no holds barred.

Times recommends

• The grim reality of life as an air stewardess
• 50 of the world's best design blogs
• Video: Georgia Jagger on fashion and her mum's clothes