I like this question as I've thought about it quite a bit. For me though I know the letter will not
1) fix things between us
2) get her to stop drinking and doing stupid stuff
In my case I feel like it wouldn't change anything and would fall on deaf ears anyhow, because what I really want is a rational person on the other end to validate my feelings. I might get an "FU", or a "your right farewell", or "other". To me these responses don't change the future and the course I have laid out, they may however bring me back to a really dark place.
IMO make peace with it yourself. What do others think?