@Legomum78 I wish you luck but sadly would recommend you don't hold out any hope of anything positive coming of this. My experience of multiple letters, summarised below, is long, sorry, but illustrates how these people will completely ignore anything but their own needs and wants.
My "mother" who never helped with my kids, but criticised them and my parenting, tipped me over the edge when she told me if I'd been a proper mother she'd have a better relationship with her grandchildren. I have refused to speak with her in person since, and set down a request that the only way we could move forward was for her to reflect on her behaviour, acknowledge it, give a genuine apology and commit to not repeating the behaviour. I explained I was too upset to speak with her so all communication needed to be in writing.
So she left a voicemail, although blocked from my phone, saying she doesn't know why I want an apology from her but she's very sorry.
Then a letter from her saying the silence was killing her (it was ok for her to give me the silent treatment for weeks on end though) and she truly meant when she left me the message that she was so sorry. But she didn't actually apologise! Lots of whining about everybody else had support when we were all stuck at home due to Covid (when I was on the phone daily and having things sent to her online).
I replied, pointing out that she hadn't apologised and showed no interest as to how I was doing, her letter was all about her. Reiterated what I said previously and pointed out some carefully worded things about her behaviour and what an apology actually is, and that I knew this would be a difficult letter for her to read.
Next letter had 2 lines, calling mine a cold unforgiving document.
I replied, saying I understood my letter had been difficult for her to read; I pointed out again that she showed no concern for my wellbeing, but asked her to get help from a therapist, enclosing details of a couple of people or to talk to someone independent like a religious person (she's "religious"). I said this was the first time I could remember asking her to do something for me.
Next letter from her said until the day she dies she'll regret the "remark" she made and didn't know what she could do to get me to forgive her. She couldn't eat or sleep etc.
So I said again I was asking for one thing, for her to speak to a professional who could help her understand what she has done. I needed her to consider did she believe what she said to be true. If so, why? If she didn't believe it, why did she say it.
Next letter ignored all that and asked me to come and see her, she insisted that was the only way to sort things out. Saying her health was deteriorating etc.
So I wrote again saying she still hadn't taken responsibility for what she said, I was seeing a therapist myself so I knew it would be hard for her because she had a traumatic family situation as a teenager, but a therapist or religious person would help her to see the bigger picture in a non-judgemental way. I needed her to understand the effects of what she said and that simply saying the word sorry doesn't work when the behaviour is continually repeated.
(In every letter I included about her saying if I had been a proper mother, and I encouraged her to show the letters to other people. Her family, without having contacted me to see what actually happened, have all sided with her and except for 1 person have cut me off.)
I got my final letter from her then, over a year ago. It started with telling me to stop lying to myself, her and everyone else and denied what she'd said. It complained that she was getting one missive after another from me criticising her like a child. She finished with telling me to leave her alone, she'd had enough.
Over a year later, she's still playing the victim, even though very seriously unwell now. I've had the odd flying monkey get in touch. None of them had the full details. None of them knew she'd told me to leave her alone. But still they mostly think I should forgive and forget and go see her. I keep saying she hasn't told me anything different and I cannot go see her because nothing has changed.
Thankfully I'm off to see my therapist tomorrow, I'm fairly strong in my stance but it's very difficult having to try to unpack what my therapist has helped me recognise is actually a lifetime of narcissistic abuse and extreme emotional neglect. My heart goes out to all of us going through this shit and I believe it is a huge, huge problem. I know for me it was only when my "mother" told me I wasn't a proper mother that the scales finally fell from my eyes. So much time wasted.