What would you do if he wanted you back?

I just found out that my bf of 2 years has been talking to another girl behind my back for 3 months. She was the girl he was seeing before he met me. We have been off and on for the past few months. We live together and I never moved out until today. I have spent a few nights at my mother's when we fight. He claims she contacted him first and wants to be with him because he baby daddy left her. He says nothing happened no making out or sex just texting and calling. He said he only talked to her because he felt lonely when we argued and when I'd leave to my moms. They have quite an extensive texting and calling record. He said most of the time it was him just trying to be nice to her cuz she wanted to come to my job...I worked at a gambling arcade....He said he gave her $200 to not show up there and cause drama. He said he dropped cocaine off to her once. I'm floored.....He has been hiding his phone under his pillow when he sleeps and her name in his phone is saved under ray short for rachel...I always assumed it was a basketball friend because he played in a leauge. So yes there were times he was texting her and I was in the same room. I don't understand. He says she's wack and a druggie and doesn't want to be with her...that she wants him but she told me it was all him and that he always gave her money to go shopping....they both claim they didn't sleep together or really hang out just call and text... idk what to think...did he cheat? He says he will do anything to win me back....I'm not sure I want him anymore....what would you do???