i am jealous of my girlfriend's ex boyfriend!?

I get really jealous of my girlfriends ex boyfriend. The guy is married now so him coming back is not the issue. But the thing about it that makes me jealous is that she kissed him and doesn't kiss me, even though she says our relationship is so much better. It makes me feel as if he was a better boyfriend, or he is more attractive than I am in her eyes, and makes their relationship then sound so much better and closer, even when they only dated for like four months. How should i tell her this?