Royalblood has a good point. Why would she not do to you what's she's doing to the other guy? And ask yourself how can she cliaim to love you, and I mean really love you when she's with another guy...by choice.
I can relate to what you're going through because I've just been through something similar. 2 years ago I met a girl and started casually dating her, but at the time she was rebounding hard from her ex bf and in a pretty messed up place with it all and they maintained a strange level of contact with each other as they played out their game of cat and mouse with me caught in the middle. She described him as abusive and eventually told me she wanted no more to do with him and was falling in love with me, despite this he was always present lurking beneath the surface and eventually she randomly cut me off, told me to fuck off and got back with him. We hadn't been officially together but needless to say it still hurt and I felt quite betrayed.
Almost a year later she contacted me behind her partners back to tell me she had never stopped thinking about me and really did love me. Shortly thereafter she left her partner and contacted me to ask to talk. She apologised for everything she'd done and told me she loved me and wanted a chance with me. She described her partner as extremely abusive and claimed to hate him and never want anything to do with him again and getting back with him in the first place had been a huge mistake she'd made when she was in a dark place. She wanted to be with someone as great as me. I expressed my issues trusting her and she told me she'd do all she could to prove to me it was me she loved and only me and I was her absolute priority. We stared a serious relationship and were, I thought, happily together, for 6 months. Then the day before my birthday she told me she couldn't be with me anymore. We talked, she was upset she asked for some time to sort herself out so she could be what I deserved. 2 weeks later I found out she was back with her ex again, she'd seen him behind my back before we broke up, restarted with him Immediatley and even moved i with him inside of a month and after I confronted her about it and she told me she never loved me like she loved him, being with me was the mistake and he has always been the one for her and moreover she had run to him claiming I was abusive and had been abusing her (not true).
With hindsight I look back and suddenly question why I would have believed that she loved me or how she could claim she did when she first came back to me when she'd been with someone else for that time. The reality is if it had been me she loved the whole time, it would have been me she chose and me she'd been with. You don't love one person but engage in or maintain a relationship with a different person. It doesn't work like that. Sound familiar?
And to put it bluntly this exprience has fucked me up. It's resulted in a reemergence of the depression I worked so hard to overcome before I met her, Ive ended up back in therapy and back on medication and it really is not worth it.
I think there are some similarities here with your story. If you choose to be in a relationship with this woman, then you need to be prepared to face and go through a convoluted, tangeled web of issues and drama that most healthy relationships don't face. And ultimately if you do try to make a go of a relationship with her, my experience suggests you're probably setting yourself up to be very hurt. Though it's your choice to make.
As royalblood said, do yourself a favour, love yourself and go and find yourself a nice single girl with no baggage with whom you can form a functional, drama free, loving relationship. That's how it should be.