crying over my bf's ex

I cannot stop obsessing over my boyfriends ex gf no matter how hard I try. I even realize that I sound ridiculous and crazy because he's with me and there's no reason to be upset but every time I picture them together and everything that happened between them I start uncontrollably crying and I feel absolutely horrible. It's gotten to the point that I can't even not stress about it. What should I do? How do I stop myself from thinking this? :(