My Boyfriend's Hidden Past!

I'm in a relationship with my boyfriend. We both love each other and are considering getting married. However, recently I found out that my boyfriend has hidden his past relationship from me. He has always told me that he was single throughout. He never mentioned his past, but I found out that he was in a 3-year relationship with his ex-girlfriend. They had a breakup which was mutual i believe because they are still in contact. He doesn't know that I already know about these things. I have also noticed that the chats and even talks on the phone with her sometimes. When I asked him who she is, he said she's just a college friend and nothing more. But I already know that she is his ex-girlfriend. I'm hurt that he's still in contact with her. I feel cheated because he has hidden his past and is still talking to her. Since it was a 3-year relationship, he surely must have gotten physical with her. Whenever my boyfriend comes close to me, I remember all that might have happened between him and his ex-girlfriend, and I feel bad that I'm not his first anything. I have asked him many times about his past, but every time he says he never had any relationship before me. Does he still have feelings for his ex? Has he really moved on? I'm confused about whether we should get married or not because I suspect he might have feelings for his ex. What if, after our marriage, he still goes to her? I'm confused.