What should I do ? I don't feel comfortable with this situation ! Help please ?!?!?

My boyfriend is going to start to go to a youth group. And there is one of his exs there that I really don't like ! And she is a MAJOR flirt! And he told me that if I want he wont go , but I don't want to be a controlling girlfriend! But I'm jealous . And I just don't feel comfortable with him going. :( what should I do?