Hey, I just got out of a relationship very similar to the one you are describing. I would say from the sound of it, her malevolent actions have nothing to do with you, but instead come from the problems from her past.

I stayed in my relationship for an entire year and it was six months of solid abuse and then I became completely dissociated because it hurt me so much. I tried to leave but she would always convince me it was going to get better but then it didn't. It was always echoes of the past for us. She would often lie to me about where she was and would often threaten to cheat and always be talking to other guys who she knew only wanted one thing from her. She tried so hard to get me to stay, but in the end I just couldn't because of all the damage she did to me. It was hard, but my friends helped convince me to stay away from her because she only ever hurt me.

In my opinion, the only thing that can help her is professional help and she's only going to get that if she really wants to. It seems to me that you are faced with a very similar decision as I was. Either accept the fact she was so bad to you for so long and live with that for the rest of your relationship while you desperately cling to the hope that things will get better and that she'll quit being borderline unfaithful, or just get out.
I chose the second because I couldn't live with the shame of staying with someone who was so cruel to me. It was so hard and took months to get out but I eventually steeled myself and she proved to me that I made the right decision when she jumped to the next thing with someone else after I told her there was no way I'd come back, not even being sure I meant it.
Right until the end she tried to tell me she was pregnant and that I'm a horrible person for leaving when she realized I wouldn't come back for gifts. I told her to take a test but she never would and we haven't really talked for a few weeks now and I'm trying to just pick myself up and move forward. Just know your worth, man, and don't let people walk all over you if you don't deserve it. Ultimately the decision is yours, but think very hard on what it is you really want out of life. Think about your future with this person and ask yourself if that's what you really want and if it's worth spending your life investing in.

You are totally free to completely disregard my opinion, but this is just my point of view, having been in a somewhat similar situation. I hope this helps. Good luck