Why does he keep trying to get back with me?

I'm currently in a similar situation. I'm in a pretty rough spot in my life right now, I had been dealing with depression and adhd for a number of years but let my pride get in the way of getting help. I had always used girls/sex in an attempt to curb my depression; but as outlets so often do, it failed to help. I then started dating this girl at the beginning of fall semester, and I've never been happier than I have been with her. if I hadnt screwed up so badly, id be proposing to her soon. things were great and I was doing well in school and everything was going well for me. then the season started to turn, and things slowly started declining in other aspects of my life, aside from the relationship which was still amazing. I suffer from seasonal affect, which is a form of depression based on the weather, and as things kept declining and the weather got worse, I slowly fell deeper into my depression until I got to the point where I really could not pull myself back up. I wound up cheating on her with a girl that meant absolutely nothing to me, a few times, and randomly kissed a few other girls. all this did was further fuel my depression; however, I couldn't pull myself out of the destructive cycle I was in. finally, everything culminated into me currently being a semester away from failing out of a near ivy-league institution, losing my work study job, and nearly losing the girl of my dreams. were currently trying to work through things and there's a lot of fighting, but she knows everything there is to know now.why I didn't, what's wrong in my life, how I feel about her.and I know exactly how she feels.i mean if you have no feelings for this guy, tell him to f*** off and move on, things are through.but if you care about him, tell him to be straight up with you about why he did it, how he feels about you, and what not.if he did it jus for the mere sexual gratification, send him on his way, but if there's a lot more to it than just thinking with his dick, consider giving it a chance

Cause its not necessarily because he was unhappy with you. Maybe he love what he had with you but was looking for something missing in another. Or maybe he just wanted the trill. The fact is he did it without thinking what an a-hole he was being and now that he lost you he misses what you afford him. Me you, don't let him back in.

Because he can't stand the fact that you don't want him anymore. Also possibly because the "you don't know what you got til it's gone" scenario.
Stand your ground. Good for you =) There are better more reliable fish in the sea.