Trust your gut instincts!!!
My advice to everyone is to never take back someone that cheats on you while your living with them, because odds are they are going to do it again. I made the mistake of taking back a guy that I dated in high school and the beginning of college that did that very thing. When we dated three years ago we both were young and naive. He moved across country to be with me in my college town and shortly afterwards he decided that he was lonely, told me that it would never work, and he slept with a coworker. He later told me that he liked her and wanted to pursue a relationship with her. We didn't talk for two years.
I heard that he moved to Florida and so when I moved back to California from the East Coast I never planned on running into him again. He ended up however living in the same town as my grandma in northern cal and when she ran into him she stupidly gave him my phone number. He called me to apologize for cheating on me and told me he would regret it for the rest of his life. He said he never stopped loving me and that he just needed one more chance to prove to me that he had changed. When we started seeing each other again, we thought it was fate, I was angry that he cheated on me, but I started believing that he only did it because he was young and that we had both grown up a lot over the past two years. We had a great relationship with the normal ups and down for a year and then suddenly he became self absorbed and went into destructive mode. He told me that I needed to seek therapy before we could have a healthy relationship because I was abused as a child and I was overacting about everything. I had a good reason though to be freaking out.. because he was hanging out with his best friend's ( since sixth grade) girlfriend everday and even enrolled in all of the same courses at the community college as her. I couldn't believe that someone that didn't want me to have any guy friends would think it was cool to spend every minute with some other girl. I started getting suspicious that he liked her and so I confronted him about it and he said "no I'm not interested in her, and I could never date her because she's dating my friend." Well I still didn't believe him so when he started telling me that I was crazy, paranoid, and insecure because of my past experiences of being mistreated, I decided to move out, because I had a gut instinct that something was going on. He then broke up with me after I moved out and told me that if I got help someday that he would want to be with me and that we could be friends no matter what. He promised he loved me and always would because I was his first love. Then I found out the less than shocking news through a friend of a friend that a week after he broke up with me the girl broke up with her boyfriend and moved out.. oh and the following week after that my ex told his best friend that he would be moving out too because he was falling for his friend's ex girlfriend of a week. He moved out and now they are living with the girl's mom until they can find a studio. Although I was extremely hurt be the news I kind of expected it to happen and I now know that I wasn't going crazy. He was such a jerk for lying to me and trying to blame me for everything. I'm much better off now. It doesn't help though that the girl tried to befriend me again when they started dating! Did I mention that this girl once was a friend and roommate of mine! Who would have ever thought that two people could be so careless about other people's feelings! Obviously they are the ones with major issues!!