Becky,
I love the letter route! That's what I did. I have to because first of all I'm a much more confident and communicative writer, but second of all, if I try to explain things to my AH, I forget important points and he starts talking and getting me all mixed up. So, the letter can just lay everything you want to say out there. It's still tough though. I cried and choked up a few times while reading it to him but I felt much better after getting everything off my chest. He was really mad. At the end of the meeting, he stormed out and said he had nothing to live for, no reason to get clean if I was leaving and 'taking his kids' from him, etc. He's calmed down now. Just be prepared to stand your ground. You have a reason that you feel like leaving him....don't let his emotions (and yours) muddle everything up.
Also good that your daughter will be gone for a week! Stay strong and keep posting!
