Duped, thanks for sharing. It sounds like you're in a lot of pain. I can't tell you what to do, as I don't even know what I would do in that situation - can't possibly know what you're going through as I'm not going through it myself. But it does sound like your ex-gf has some drug and alcohol issues that may be complicating things and changing her personality/who she was before.
I think maybe, at this point anyways, you should just give her space and hopefully she will work on her drug and alcohol dependency problems and mental health. It's amazing how helpful a few sessions with a good therapist can be. But that is not something that will solve a problem right away. It's going to take time. And you have to decide if you are willing to wait for her to heal - if she works toward it at all, sincerely. Especially if she is going out with other people.
I would just give it a lot of reflection and go with what your gut tells you is the right thing to do. Good luck.