From a different perspective (but similar experience)...
My ex was an addict / had depression. We went through many cycles of him trying to get/stay clean and me trying to save him. When we finally split it was because he got clean and realized that he needed to be on his own for awhile in order to work on himself. It was a mutual decision, but a sad one at the time. But with time I came to realize that us splitting allowed each one of us to do some work on ourselves, and it also left us open to finding other people to whom we were better suited. (we had other issues) Yes, at the time, it made me sad, but in retrospect, I wouldn't have had it any other way. He was a good person and it was sad to see him saddled with two diseases to battle; when I finally accepted that I could not help him and he accepted that I couldn't, we were able to let go and go out separate ways. I think with the addicts I've known they sometimes have to separate in order to get better.