HE CHEATED NOW I'M CHEATING

We all have issues like my smelly friend buy I think I did some confessing myself. I'll start from the beginning so everyone can get a good idea what has happened. I met b/f through a friend of a friend. We started dating, I knew his first wife, their son and my son practically grew up together. He married again but that marriage didn't work, I met him while he was seperated with his second wife. He was seeing his highschool sweetheart but broke up with her when we met. We were getting serious then he screwed it up by not asking me to a banquet (baseball) and never came back to my place like he said he would. He ended up losing his keys and going back to her place. I left him after that because that was the 2nd inncident regarding his highschool sweetheart. Then he calls me and tells me his second wife died who he was seperated from. To make a long story short, we got back together, we moved in together but something wasn't right. I ended up hacking into his email and following him. He was having something going with this girl from work. I ended it with him and he kept coming back and I kept taking him back and finding out again it never ended. She had my email and when she was pissed at him, she would email me and tell me what was going on. She would tell me that he told her we were not together anymore. Even after 17 weeks of counceling, he told the councelor that he was not seeing her, meanwhile he was. The last straw was when we broke up and he ended up introducing her to his family, I was devastated and finally ended it. Only to have my boyfriend not leave me alone. I hated him and didn't want to go back to him. I needed some closure so I got a hold of the woman who has been carring on with him and she told me they went to a funeral and he was so distant with her, she told me that he started to cry and realized how much he has hurt me and how much I never deserved it. He told her that it was over with her and he can only hope that someone day I would forgive him. When she told me this, I really didn't want to take him bake but I did. He has been 100% different the past 2-3 years. But, I met someone through my friend and he has a girlfriend who he doesn't sleep with ( i know this to be true because I know someone who knows her) they have a 3 year old but really don't have a relationship, she goes away almost each weekend to be with her sister and he stays home and that is when we get together. We really care about each other and he knows about my situation and I know about his, only now I'm starting to feel guilty (it's been 8mths) for the affair. I'm no better now then what he has done, I was faithful the whole time he was not and never once thought to stray, why did I now? part of me wants to be alone now, I don't want to leave him for anyone but what I think I need to do right now is be alone.