How can I stop crushing on a guy that I can't have? I have never felt this way before...

There is this guy that I have known since we were little kids, he has always flirted with me but when the opportunity came up, I rejected him. I felt that he was too young for me at the time and thought that all he wanted was sex. I ended up in verbally abusive relationship that lasted almost 3 years. He also got into a relationship and they had a baby together. He and I talked the other night and he said he was unhappy in his relationship, that he tries but he's giving up hope and knows that it won't last. I told him I regretted passing him up and others as well. He told me he would help me pick out a good guy but would still be kinda jealous. We laughed and talked for a couple hours. I seen him again last night and we talked for a few minutes. I have never liked anyone who has a girlfriend or wife, no matter what because it's wrong but I can't stop thinking about him. I know we can't be together so I need to stop crushing.