My (x?) abf is currently in a hospital for psycological testing and will be going to rehab when he is discharged. I did similar things with encouragment and love. Obviously that did not stop him from binging and ending up 1000 miles away with no money, no food, and sleeping in his car. When he needed help he reached out to his xgf because she enabled him for 7 years and he knows he can count on her to bail him out.
My point is that I think these letters/ messages/ conversations do more for us than they do for them. If we put into words how much we love them and are here for them, then we won't feel like we gave up on them and we don't blame ourselves (and we shouldn't). So do what you need to do for yourself, but keep your expections of their reaction extremely low.
Hugs.