I'm jealous of my boyfriend's ex. how can I get over it?

I've been with my boyfriend for two years. I can't seem to get past his relationship with his girlfriend before me. They were together almost 7 years, and they had a bad breakup and she left my boyfriend for another guy. I've never even met her, and he doesn't even speak to her anymore. However, I am so jealous of her. I look at her facebook and myspace and I wonder why he even was with her. I don't know why I feel like this. They had a totally different relationship than my boyfriend and I have. They lived together, and didn't go to college so they could snowboard and screw off. I finished school and I work hard to support myself, and I've never relied on my boyfriend to support me. I don't know why I am so jealous of his past girlfriend. How can I get over this?