Why can't I get over him?

So basically, about 6 months ago, I met this guy on a gay social networking site. He messaged me and "friended" me first, and he acted like he was interested in dating me. He made me feel happy, and we went on a date. The first date went pretty well, I thought. So we scheduled another about 2 weeks later. It was a disaster. The date was canceled early when he found out that he needed to study for a quiz the upcoming Tuesday. After this date, he completely stopped messaging me, and I ended up having to start the conversation. The last time we talked was back in May or June.

I did get over him eventually, but about 3 weeks ago and a week ago, I went to a nightclub and saw him there both times. He was with another guy and looked really intimate with him. This made all those happy times with him come flooding back, and I immediately felt like crap again after seeing him there. I told my friend this, and my friend told me that the guy I dated was known to be a playboy.

The question is: why can't I get over him? And how can I get over him? And despite what my friend said, why do I still think that it was me that drove him away when most likely, he just wanted sex and left me because I wasn't giving it to him? We never had any sex on those two dates; the farthest we went was kissing and holding hands.

Just recently, I took a midterm and did way worse than expected. I would definitely say that it was because of the abrupt end to our string of dates and whatever ties we had. There was no closure, and I think that bothers me the most. I know that I can't let him get to me because if I do, my grades, my happiness, and my health will all suffer, but I still just can't get over him for some reason.

Any advice on how to let go? Thank you very much.