My Ex slept with a guy one week after we broke up, lied to me about it, and 5 weeks later wants me back.......

My girlfriend and I had been living together for almost 3 years. Recently things had kinda gone sour because we stopped communicating to each other. I was busy working 6 or 7 days a week and wasn't giving her the attention that she needed so she began to go back to partying all the time which made me resent her. She had this guy friend that I was weary of just because he would text her and stuff that made me understand he was trying the "back door method" (we all know guys try to be friends then try to get with the girl when the time is right). Anyway she decided we should take some time apart she was to move out. We were offically on a "break" but were "still together". She got really drunk at a small get together at her house and I came across pictures of her topless and kissing this guy. I obviously was enraged and immediately broke it off with her, I said I would never consider getting back together with her. She then screws the guy for 2 weeks, lied about it and now wants me back