Dating a girl that has cheated and walked out on her kids?

For the past 3 years I was seeing a girl who has cheated on me at least 4 or 5 times that I know about. When I met her see didn't even have custody of her kids because she walked out on her huband and 2 little kids. Since she got back 50/50. However, when I met her she was very sweet and thought the world of me and wanted to marry me and have another child. However, after about 6 months was the first time I caught her cheating. She begged me to give her another chance and I did. Since then she has broker up with me at twice to see other guys and then after a few weeks, came back. Now she broke up with me to see some biker (gang member) and I know I should be happy she's gone, but I can't help but feel sorry for myself. Will she ever be able to be faithful?