Those are some chicken-or-the-egg questions you have there.
Alcohol is a depressant. Not all people who are depressed cheat or become addicted to alcohol. Not all alcholics cheat. Not all cheaters are depressed or alcoholic. Who knows. You could spend a lifetime trying to untangle his knots, but unless he's trying to untangle them himself, they will only get tighter.
Meanwhile, you have knots of your own to untangle.