Try to understand that does not allow you to forget about the ex-girlfriend of her boyfriend. For example, if he talks about her and does it with irritation and even anger, then you most likely cause for jealousy there, so calm down and don't think about it. Perhaps in past relationships, the girl he was greatly offended, and he can't let go of the negative feelings. But if he remembers it with nostalgia, then you need not be jealous, and to think, he loves you. Talk to him on the subject, if he does not change behavior, the best option is to end the relationship.
If the ex-girlfriend of your boyfriend constantly reminds me, it would be difficult not to think. But in this situation there are two options. It happens that people from one company break up, but continue to communicate. In this case, do not roll up hysterics and scenes of jealousy, but better to look at her. If a girl is really acting like a friend, don't mess with her. But with the blatant flirting and belittling you in the eyes of the young man, try to talk to him and explain the situation. A reasonable person he will correct the situation and will not give more cause for jealousy. But if he will resist your words, think about it, maybe his feelings have not cooled down.
When the annoying behavior of girls, the constant pursuit of your a couple or a single young person can hardly see his guilt, so jealousy makes no sense. Better support a loved one in this situation. Ignore the annoying person, time will pass and she will calm down. Such men rarely come back, so the concern you have.
If your boyfriend never remembers my ex-girlfriend, and she doesn't appear in your life, do not be jealous to the fact of its existence. After all, she is his past and you're present and future.