"How do I come to terms with why she has done this and how do I help her?"
-You are an awesome person for asking these questions, heroic, even, for it takes a heroic person to deal with love and betrayal of love and still have such a high level of respect for the betrayer.
-I was taught that I cannot control anyone, that I could only control my actions and reactions to the things that people did to me. I feel that once a person truly accepts that in their core of being, then life becomes so much easier.
-This person is dealing really dealing with one sickness: addiction. The only thing that I can see you doing is making positive suggestions and providing healthy alternatives. Beyond that, "the ball is in her court." And that is going to be hard to live with.