jealousy

I'm having trouble getting over my boyfriends past partners. He only had two and I know he doesn't care or think about them but every time he brings a certain thing up or I realize that where we have had sex he has also had sex wth them it just makes me think about it and become sad and make me not want to have sex with him. I know it's not right and it's not his fault I just don't know how to get over this feeling. I love him and talked to him about it and he is trying his best to help me but he and I both know I have to get over this feeling. I just don't know how. I just need some advice and to talk to people who have been through the same thing.