He cheated on me, now I want to die?

Lately I was dating this guy for the last 2 months. Things had been seeming to go well. and then he kept getting weird about the commitment thing. He started to get freaked out when we started seeing eachother a lot...and then things got more serious. I started getting annoyed by his behavior and didn't know what to think about it. Always pulling me close and pushing me away. At one point I told him I needed a little bit of time to think about things, and he wasnt happy about it. I tried to apologize but then he just got really angry. Eventually to my dismay, I find out now hes been seeing some girl behind my back for one month. That explains how distant he has been in the last month. He was seeing her behind my back and I had NO idea about it. I am so hurt and the last couple nights i couldn't even sleep. I thought he had feelings for me. And only me. That's what he told me...I talked to his sister though after this happened, and her only response was "yeah thats mac." I guess he does this type of thing repedatively with different girls. And never settles for long. So this new girl I have no idea what shes like. But i dont know i feel like hes not going to do it to her, and I was the only one taking his crap. Can anyone offer some comfort? I don't know why hed go behind my back and do that.