My best friend is cheating on her boyfriend with me?

I know..we're bad people...at first we used to just cuddle and kiss on the lips which seemed pretty innocent to us then. We didn't find it a big deal because most guys don't mind lesbian stuff. But then our relationship went to the nest level and we started making out and doing a whole lot more...but she's dating this guy who she says she's in love with, and if he knew what we did he would be totally angry and break up with her. I slowly fell in love with her...if I knew that that would happen I would of never kissed her in the first place, because it seemed like just fun at the beginning. I felt really jealous of her bf and I feel like a side chick...which I basically am. I tried getting over her by getting a boyfriend, and he knows everything me and her do and is okay with it. I'm not really into him that much though and we're in a pretty casual relationship. About a month later she confessed that she was in love with me and I told her I was in love with her too. She said she was sad because we both had boyfriends. A few days later she brought it up again and I asked her if she's in love with her boyfriend too...she called herself a bad person and said she was in love with both of us. I don't know what to do...I don't want to stop talking to her because she's my best friend, and I kind of don't want to stop doing things with her either...it's hard to resist and I'm deeply in love with her. But I don't like the fact that she's dating this other guy.