How can I stop comparing myself to someone else?

I keep comparing myself to this girl I went to school with. In school she was an outcast, never cared how she looked, crazy thing is we were even friends. But we became enemies when she messed around with my boyfriend. Now she looks a little better and has a facebook that I keep looking at. Everyone tells me she is average, not that pretty, but for some reason I think she is pretty now and skinny ( she lost weight ) . She has a great job, rides a motorcycle, she's single and goes out with her friends,and of course all of this is on facebook. I on the other hand have gained 20 pounds in the last 3 years, have two jobs and am still struggling, married with kids and I want to stop comparing myself to her but how do you stop? I mean, there's just something that makes me keep thinking about it and looking at her facebook. I hate her now, and I don't want to. What I want to do is feel nothing, just not care. But I don't know how. Help