What they all said, it'd be a terrible idea.
Have you done any therapy/freedom programme etc?
If you haven't, please know that you won't heal from the abuse, only bury it. Sooner a or later it'll come out again.
Like it is now. Take charge of the past, your feelings and thoughts. Deal with it now, when your happy and safe and you'll never have to deal with it again,
Bury it now, and it'll stay unresolved and could come back again to haunt you.
You're going to be alright, but it's time for you to process what was done to you.
Write the letter, go scream it from a hilltop if that helps, but never, ever, ever contact your abuser again.