There is no easy way to get over a broken heart. No advice from the wisest psychologist will make you hurt less.
Take pride in the fact that you did the right thing, though. I was stupid enough to marry one of these asses. The sooner you get out, the better.
I think that you have a hard time to let go because he made you feel that you could make a difference in his life. After all: while you were in a relationship he actually stopped the drinking and the drugs. Things like this give one a sense of self worth. (Experience speaking) If you stayed you would have had nothing to look forward to but one big emotional rollercoaster.
Good luck, Sweetie. You are in my thoughts.