"Write a card to Peter. Say sorry from Mary."

Who's Peter? I know everyone Mum knows, I write the Christmas cards every year.

"My first boyfriend, before your dad. I need to say sorry."

My mum has MS. She's had it since I was five. Early on it was simple. I could explain it by saying, "It just means she can't walk," but not now. Now it's worse. It progresses slowly: plateaus and falls, never improvements. Mum's a fighter though, still smiling at every stage. Her motto is: "You've just got to get on with it, what other option is there?" That's the best thing my mother's ever told me.

Nowadays she's for the most part bed-bound. She can only move her head. We do go out sometimes, but she finds it exhausting. Mum's mind's still there though. Sharp, sarcastic and mischievous, she still has plenty to say. The problem is getting the words out. As the day goes on, her voice goes too. This has happened slowly over the past few years. So slowly, you don't really realise.

The mornings are best. I can understand for about half an hour, but then she gets tired. The rest of the time, I get there through guesswork. I know how her mind works; I can anticipate what she wants. But not with new things. New things are difficult. Early in the day I might get an outline but details are near impossible. So much to say, without the means to say it.

So when she asks me to write a card to Peter, her first boyfriend, you can imagine my confusion. She definitely mentioned him before, years ago. I know they worked in a hospital together, and that he had suffered burns on half his face. I ask Mum how long they were together. Four years, she says.

But why did she want to say sorry? What did she do? Did she cheat on him with my dad? I asked her. She says no. She met him when with Dad but the rest I can't make out. But whatever it is, she felt the need to say sorry some 28 years later.

I was reluctant to write the card. Firstly, where would we send it? Amazingly, Mum remembered the address, the address he had when they were together. It takes half an hour for me to understand it, but we get there. Secondly, what will I write, and is it a good idea? What if he has a family now? Won't he think this weird? Should the past not be left alone?

The week before, Mum had suffered a chest infection. It was touch-and-go whether or not she'd make it. You can't really say no in a situation like that. And she was adamant.

So we wrote the card, a short note from Mum saying: "I just wanted to say sorry for hurting you all those years ago. Hope you are well," and a little note from me explaining that Mum had been unwell and that this sorry note was something that seemed important, and I hoped he didn't mind. We didn't include our address. For Mum, it was enough to send the card. It was only a gamble that he'd even get it.

But somehow, it did make its way to him.

The mystery surrounding this card made me think. Why didn't I ask more questions before? Why didn't I listen to the answers? Why did I sulk moodily when my parents took me to Bristol and showed me the places they'd once lived at, danced at, kissed? At 13, you don't realise. The last thing you want to know is anything about your parents. You want your own life, certainly not theirs.

Four weeks after posting the letter I'd forgotten it. Mum's never mentioned it again. But Peter's tracked me down. I have a message on Facebook from Peter Osborne: "Is your mum's name Mary? If not, please ignore this." I've seen his photo; it's a bit blurred but you can still make out the burns. I know he studied politics at Glasgow and now works as a sculptor. He's a member of a number of socialist groups and has friends called Mandy, Alan and John. I know things about this man that maybe my mum doesn't know. I could ask him why she said sorry. I could find things out.

But I won't. I'm not replying to his message. I thought about it and I don't think I should. Mum has said her sorry. She hasn't mentioned Peter again. I know there are things he may want to say, to accept the apology or ask questions. But Mum chose not to include her address for a reason. She's made her peace, and I don't need to know. Peter is Mum's past, not mine. I know the important things; I saw for myself how much she loved my dad before he died.

I remember the things Mum told me when she could, the things she repeated, the things that meant the most. Anything else would be a luxury. There may be questions left unanswered, but you can't know everything. Sometimes the things your mother never told you must stay that way.