what do i do??

my ex and I have been dating on and off for two years now. He's cheated on me, started horrible rumors about me, and messed with my emotions. He's said that he loves me and is sorry for SOME of the things he did. he's dating someone now and told me that they had sex and he loves her but can't get over me and still loves me. I was so pissed off that our best friend (they became friends while we went out) has asked me out and I said yes. I'm so confused and need help/advice.