You didn't cause it, you can't control it, and you can't cure it. You can, however, contribute to it. The goal is to not contribute to it and you seem like the kind of person who can meet such a goal (if you can get over your romance novel view of relationships).
Good luck!
Cyranoak
P.s. Just as an FYI, damn near every one of us has a significant other who is "the best most wonderful person when they aren't drunk." We all fail to remember that they are alcoholics and drunk much of the time. We all fail to remember they are the total person, not part of a person. Hell, I'm the best most wonderful person when I'm not trying to control and manipulate everything to happen my way. Just ask my wife.