Hello musiclady. I'm so sad you're going through this awful time.
I've been on both sides of this thing. I lost my husband in the same way - I simply couldn't reach him & he wasn't listening or comprehending. Our son was only 4 when I had to give up on trying to help. I was the classic enabler, but didn't know it at the time. The company he worked for sent him to re-hab & when he got out he said, 'Well, I'll still have a beer now and then'. My heart was broken, since I knew we'd be going back to the same old nightmare of him trying to control it.
Many years later he admitted how he grieved over losing me. Sadly, it was way too late by then. I agree with the others - until the person wants to listen & receive help, we have to save ourselves. I do like the idea of a letter though. He can read it many times over & maybe some of it will get through to him. You can say what you want without being interrupted. Glad you are here - please let us know how it goes.