I'm jealous of my boyfriend's exes
Why are you jealous of his exes?
Because they once had something that is mine.
Being jealous of who your partner has been with in the past is somewhat self-defeating behavior. Your partner is with you and not them and so you shouldn't have doubts or compare yourself with them. If this continues to bother you, you should discuss it with your partner and address any issues that come up.
Yeah, but other girls have done it with him and it bothers me.
That's true, but remember that your boyfriend chose you.  His past relationships didn't work out, but he is trying to make it work with you.  Be proud of your relationship, work to make it stronger, and you won't have anything to worry about.