How to sum up my 19-year-old daughter, Rosa? A “free spirit”, a “one-off”, “a character”, “she lit up a room”. These were the phrases most frequently used in remembering her after she drowned, following a seizure while she swam on a warm July morning in 2008.

She was indeed a one-off. Fun, irreverent, stubborn, uncontainable, loving, garrulous, challenging, ingenuous. She could be moody and withdrawn, too, but she always said exactly what she thought – sometimes to my acute embarrassment when she revealed things I had voiced in private that I didn’t want repeated. The doctors attributed her “disinhibited” behaviour to the way her brain was wired because of her epilepsy. She never thought: “Oh, I’d better not say that,” she just came out with it.

Eight years after her death, I ask Adam round. He was the boy Rosa loved while they studied at university in the 10 months before she died. They met on a dancefloor after the first day of term and had been planning to live together in the next academic year.

Now Adam sits opposite me. His asymmetrical haircut has been replaced by a shorter one, his features have matured and his gaucheness has gone, but it is still long, lean Adam with his puckish face and northern inflections. He plays me a trio of films he made of Rosa during that student year – skating, mucking around with friends, out at an amusement park. They were films we had never seen, and which my husband, Andrew, still finds too difficult to watch. “How can you look at them?” he asks me. It is because I need to uncover, to know.

Vanessa and Rosa.

As Adam and I talk, it is painful to learn about the things Rosa hadn’t told me, and to see how she behaved when she thought her mother wasn’t watching. And it is hard to listen to Adam recollect how he became involved with Lucie six months after Rosa died. Lucie is the girl with whom he is now travelling the world. But I don’t want to avoid hearing. I want to know everything.

I set out lunch; there is a pause as we begin eating. Then I find myself apologising for a truly bad thing I did when Lucie and Adam got together.

“I’m so sorry, you know, about that terrible thing I did.”

Adam nods. He knows exactly to what I am referring.

I try to explain. “I had these moments when I thought I would go insane with the grief of it all. It was overwhelming. The problem with grief is that you are in that moment when the other bereaved person – or someone else – isn’t in the moment. It’s very hard to find the times when you can talk about it and be supported and supportive. Andrew and I …”

“What are these?” Adam interrupts, pointing to the various dips I have placed on the kitchen table. Would he rather I didn’t talk about this any more?

“… Andrew and I found it hard to share times when we could be supportive of each other. So I would have these private moments when I would be a bit obsessive and sit and watch that little film you made on holiday of Rosa in a bikini mixing a cocktail, or I would go on Facebook, rather manically, because I still had her login details. They were registered on my computer at home and it was logged in all the time. I would go to my computer and look, pathetically, at what her friends were doing, hoping they had left a message saying they missed her. It was a way of connecting with her, of still being part of ongoing lives associated with her.”

I was on Rosa’s Facebook page when I saw a conversation between Adam and Lucie that made it clear they were in a serious, loving relationship. It was six months after Rosa had died. I instantly felt the most unimaginable pain, as if my daughter’s life had meant nothing, as if she had already been replaced.

With the grief came overwhelming rage, the “It should have been her” scream. Before I thought through what I was doing, I had posted: “Don’t you think it’s a bit soon to fall in love again?” on the stream of their conversation and pressed “Done” or “Send” or whatever it is.

Immediately, I panicked. I knew it would come through to Adam as a message from Rosa, his dead girlfriend, with a photo of her attached. But I was frozen in front of my computer. I seemed incapable of doing anything to put it right. After a sleepless night, I sent Adam a text. “I’m so, so sorry, I’m so ashamed.”

He was understandably furious and upset. “That was terrible. You don’t know what I’ve been through,” he texted back. I kept apologising because I was truly, deeply sorry and ashamed.

Once again, I am apologising.

“I shouldn’t have been looking at your private messages. But I was in this weird state of needing a connection with Rosa and, seeing what I saw … the grief was overwhelming … it felt as if everyone was moving on and I was stuck. Iit was like: ‘Rosa is gone, I want everyone still to be grieving for her.’”

Then I tell him how I hadn’t slept that night, imagining him getting this message out of the blue – from Rosa. Adam hasn’t said anything yet. “I felt dizzy,” I continue, “and physically sick. And mortified. But it took me hours to pluck up the courage to confess and say how sorry I was. It was like when you have done something terrible when you are little and not being able to tell your parents. It was awful, and I can’t imagine how terrible that must have been for you.”

“It was so freaky …”

“I’m so sorry.”

“… I’m not religious; I don’t believe in anything superstitious. I can understand why some people believe in it, but I’m from a science background; those sort of things don’t make sense to me. And then that happened and I was …”

He pauses. “It was almost like someone had found out something you never wanted them to know, and now the world knows. A wave of fear came over me; I kept thinking: ‘I don’t understand how this could have happened.’ There was no logical answer for it. I felt so sick.”

He then tells me it made him question the relationship. He suddenly felt really guilty about meeting Lucie.

I am repentant, contrite. Chastened. It is my turn to be silent.

“I remember just not knowing what to do?” He throws this out like a question.

“Did you tell Lucie?”

“No, no, I didn’t tell anyone for a while. I didn’t sleep that night either. It brought everything back.”

“So, at what point did you think it must have been mad Vanessa? Did you think: ‘Oh, that bitch of a mother!’”

“No, I didn’t know it was you. It could have been Andrew, [Rosa’s sister] Ellie, you …”

I interrupt. “Andrew or Ellie would never do anything like that. And, looking back, it is incomprehensible that I would do that to you. It shows what forms grief can take. I had told you to move on with your life, but I couldn’t bear it when you did. I tell people this story. It’s almost like a confessional; I have to test my friends: ‘I’m going to tell you something I did that was so awful, will you still be my friend?’”

“There was no need to hold it against you. I know you were upset.”

“I think I felt you didn’t love Rosa any more because you had transferred your love to someone else.”

Adam smiles. “You’d done a Rosa! You jumped in without thinking of the consequences. That’s where she got it from!” Adam laughs. I join in, but then I become more thoughtful.

“I do share the recklessness that Rosa had. When I write, I need to say things that maybe I shouldn’t share. But at some level I don’t want to be sensible and careful – I want people to know how it feels.”

Yes, I want to bring them up from the basement. Expose, remind, confront. It is risky.

Adam is quiet now. We have come to the end of that part of the conversation. I hope he has truly forgiven me.

• The Truth Game by Vanessa Nicolson is published by Quartet, £15. To order a copy for £12.75, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p on orders of more than £10, online only. Phone orders min p&p of £1.99.