Me and him will never be together, why can't I get him out of my head?

When I was 14 I met this guy and we had a one night stand. A month went by and I found out he was trying to track me down, so we ended up getting together and from there we started dating for a year. He was 8 years older so there was a lot of issues with our relationship so I ended it. A year or so later I was having a bad day so we got together and hung out and he confessed that he still loved me and wanted to be with me, but I had a boyfriend so I told him it wouldn't work. It's been 2 years or more and a few weeks ago I ran into him on an old MSN account, we started talking and it brought back all these old feelings, and new feelings and I realized how badly I wanted to have a relationship with him now that I'm older and he's more secure. After a few days of talking I realized that he just wasn't the one for me, and that he was an ex for a reason. In the 3 years we have been broken up I have gotten engaged and had a beautiful baby boy. When I told him I had a child he lost all interest, said maybe one or two sentences and just left. He hasn't talked to me since and I doubt he will again any time soon if at all, but I can't get him off my mind. I don't understand why. What can I do to get it through my head that I don't want to be with him, and that he isn't worth a second thought. Sorry this is so long I'm just so confused.