How do I overcome this??

I am so obsessed with my appearence that I feel as though it is destroying my relationship. I think it's safe to say that I dedicate 90% of my life to the way I look; shopping, makeup, hair, checking myself in mirrors every 10 mintues etc.. My boyfriend and my past boyfriends have all claimed to have fallen in love with me and told me how beautiful thye think I am etc... But if I see another pretty girl around, I pretty much have a minature breakdown, BELIEVING that my bf wants them more and if given the chance would dump me on a whim. Obviously, it is a self esteem issue.. but I just don't know how to get over it. No matter how good I think I look I'm always so paranoid that my bf will find other girls more attractive. I won't even go certain places with them if I know a certain girl is there, at risk of him checking her out (which would throw me into a frenzy). How can I deal and get over this?