Why am I still jealous of my boyfriend's x girlfriend?

My x boyfriend's gf was my friend before I didn't know thay they were x's. But I was the first gf of my boyfriend and that was 2004 then when we broke up he went to australia and came back here and had a nother gf year 2006. It was a small world that I met his gf in my company that girl became my friend which is I didn't know that she was the xgf of my xbf. Year 2007 my x bf came back to me and we realize that we still love each other so we started again. After that I introduce my friend to him...that girl...his x gf. They were so surprised an I was too. After that, I can't stop comparing myself to her. I always remember how she tells me about her x bf that that was my boyfriend. I'm always hurt everytime i see my boyfriends face. I can't accept that that girl had been his part in his past relationship. So we always fight. Though he's trying to show how much he loves me I still have that feeling of insecurities. Why I feel like this I hate this feeling I feel so angry to my boyfriend though he didn't do any harm on me. I can't accept his past about that girl. Why I can't forgive him though he didn't do any mistakes. I feel so heavy. Please help me....please..