I don't have any really good advice,, but if I was going to do that, I'd be myself and speak my mind in full.
The thing is, I won't be writing one. I figure if no one listens to me and gets it when I am alive, they sure aren't going to listen to my words or get me when I'm gone. Plus, I think it's nearly impossible to explain pain so intense that a person needs to ctb to alleviate it. Most people are never in that position, so they don't understand those who are. All the words I have to explain it are not going to make them understand.
But I am most certainly not trying to tell you not to write a letter. Whatever you need to do to bring you peace, which I hope you find no matter what road you take.