@Lauraadelesmum Keep hold of that letter just for now, and start with a fresh one. I totally and completely understand how all these things have been eating away at you for years and years and years, how there's so very many things you want to tell her, and how it hurts like the depth of hell not being able to do that. I genuinely do understand.
It's a fcuking rough situation for everyone involved and there's no easy fix sadly. You're not going to like what I say now, but I promise you it will help you eventually.. If you want meaningful contact, to be a part of her life, you've got to accept that at least for now your feelings mean absolutely nothing, they're worthless, and you need to shut up about them.
Not because you're in the wrong, or you don't matter, or because I'm trying to make you feel bad, anything like that - just because you're her parent. Makes no odds if we're mum or dad, sometimes we've just got to take it on the chin and be grateful for the chance to do so. Imagine your baby's having a really bad fcuking day, everything's gone to shit, she's in trouble at work, crown prince Abdab's giving her grief - then she opens up the really long full on letter from you talking about all kinds of traumatic things from her childhood - it's not gonna help her, she won't read it, and she'll probably get upset and angry, throw it in the bin, it'll convince her she's doing the right thing not talking to you.
Try writing a short, light-hearted, positive little note and go from there. Go at it like you would if you were trying to chat someone up down the pub, make it short, maybe funny, be certain to mention something personal you've noticed about her (do you still like x food, is it good there? what's the tv like, do you still watch x program?). Be sure to leave as casual, easy to answer, open ended question about something she might like to talk about. DO NOT say anything heavy, controversial, or that'll make her feel guilty, or that implies you're upset with her, nothing that'll make her feel pressured or obliged to respond. Make it fun and engaging, something she'll want to respond to.
Hope this helps :)