Write what you feel. Then put your letter away for a day or two. Take it out, re-read it, revise anything that you think needs to be improved or made clearer. A first draft is never as good as a revised one.
Tell them that it's your decision and yours alone.
I suggest that you don't blame anyone for anything, even if you think that other people bear some of the responsibility for where you find yourself now. Blaming people, even if justified, probably won't make the world a better place. That letter will be your one last chance to make the world a little better than it might otherwise have been. Don't waste it.