Dating a girl who left her kids?

For almost 3 years I was dating a girl who left her kids. What happened was she walked out on her husband and 2 and 4 year old for her friend's husband who she was having an affair with. that lasted about one week. She moved into her parents house and started to date various guys. I met her a year later and she really tried to clean up her act. She got 50/50 of her kids and things were going well. And I decided to give her the benefit of the doubt. Then she I caught her cheating on me as well, 2 or 3 times that I know of. I love her very much, but she got into drugs (which I don't know if she's still on), losing money hand over fist, and became even more wild, I even think she might of hooked up with my friend, Last I knew she wanted to be friends and still see me, but she's seeing some biker gang member (most liely her flavor of the month). I know this girl will try and get back with me, she even dropped off something I gave her a long time ago (that I have no use for) just yesterday. I know I need to be strong a just ignore her when she comes around again. But it hard for me. And its not like i'm a bad looking guy, I think I may be too nice.... anyone have any ideas?