Hmm. I guess I can give my experience. When my first serious gf and I broke up, I didn't get affected by this too much and I got over her really fast.
Interestingly enough, this wasn't the case with a girl I went out with for a very short period of time. When we broke up, I was a wreck. I couldn't function at all for at least a week, I was depressed for a few months, and I was pissed off everytime I saw her with her new bf. Eventually, she broke up with him, and the girl and I met up at a friend's party. We both got kind of drunk, and she ended up at my place (nothing sexual happened unless you count some kisses). Now we're just friends.
In your case, I highly suggest just cutting off all contact with your ex. The more you have contact with him, the more you will think about "the good old days". Some people can't "just be friends" with their exs.
This is starting to get long and boring. If you want to help yourself forget about your ex, keep yourself busy as hell. Between school, sports, and the tiny social life I have, I don't have much time to think about my exs. Take up some activities, do something you've always wanted to do
*Edit after seeing above post
I agree with the above poster. Sit down and be honest with yourself. Why do you still think about your ex? Is it because you want to get back together? Or is it just random thoughts? I still think about all my exs, but none of them are thoughts of getting back together, so I think i'm ok
