Why am I still hung up on him??!!!?

So, me and this guy dated (i was 14 and he was 18 at the time) We dated for about three months. i know that's not a long time. but we had . he was my first and he persuaded me so I did it. bad judgment on my part. So one day while making out on the couch he said, "come on Becca I know you have more tongue then that" well...that's okay but...my name is Mary. Becca is my best friend's name. and then I went through his phone and he was ting with my best friend becca. and One day my, him, and becca were hanging out and I went to bathroom and came back out and saw them kissing on my couch! so i finally dumped him. becca and him have been going out for abut six months now. I haven't talked to either of them since. so after all he's done, why do i still want him back? I keep telling myself its just because he was my "first." but i don't know. help?! he was also my first serious boyfriend and i was young.