I have to ask again – didn’t it bother you that she had a boyfriend? You’re playing with fire here. She’s still got feelings towards this guy, the man she’s betraying by cheating on him with you. Sounds to me like maybe guilt is getting to her. Or she’s having second thoughts.
Aside from the trouble you’re asking for by knowingly becoming a vertex in a love triangle, you have to consider that if she’s willing to cheat on him, what’s to keep her from doing the same thing to you a year down the road?
Something bad is coming around the corner. My advice would be to bail. Relationships are tough enough without this angle. Seek out girls without this kind of entanglement. They’re out there.
EZ