Jealousy

So I am currently in a long term relationship with a man that I am head over heels for. But I have some serious problems that root back before his time, meaning I have serious jealousy issues. I don't even understand why I have them because I know I'm fine as hell, and he would never cheat on me but whenever he talks to another girl and jokes with her it makes me want to cry. I can't help feeling like he could do better than me, and it makes me sick. He obviously loves me because I've recently expressed my feelings and he's helping m work on them. Except he makes it seem like there's something wrong with me. Now see, I'm not jealous all the time. Hell he can talk to any girl he wants. It only bothers me if she's prettier than I see myself or if she's skinny (I've struggled with eating disorders in the past and I'm still very sensitive about my weight and I do compare myself to others too often which is wrong). But any ways, long story short... Is it wrong to be jealous in situations where he's talking and joking to a pretty girl right and making fun of her in a somewhat flirty way? How do you get over these feelings? I want to stay with him, I love him but sometimes it feels like it'd be so much easier to not have to deal with jealousy at all, and be single. I'll obviously have to overcome it sometime so I pick now. Please help!