Dear mammabear, I think I can feel how really desperate you are to end this whole alcoholism nightmare that you have been living in. I suspect that most everyone on this board have been in your shoes in one way or another.
Are you hoping that this will shake him up enough that he will agree with you that he has a problem and promise to stop? Because, he might do that. That is one possible reaction. But, it doesn't mean that he has the ability to keep that promise--not for very long. Total abstinance with a really vigorous program---sponsor, working the steps of AA, maybe an individual counselor, in addition, etc......is the only thing that can stop the alcoholic progression. The first year is really hard for most and the partner also needs a program of recovery in order to cope.
I guess, I am really asking---what are your true expectations if you give him this letter? Are you able to take another round on the roller coaster if he asks for one?
I am just saying that these are questions to ask yourself. Just be honest with yourself about your true motivations before you make your decision.
When you do decide to go---remember it is your decision, and you don't have to defend it or explain it.
dandylion