Mermayd, your situation sounds exactly like mine, only reversed. I think I can give you some perspective from your boyfriend's point of view. You are acting just like my boyfriend in our situation. When my boyfriend and I first began dating, I was just getting out of a two year relationship. I was still talking to my ex a bit because he seemed very upset over the break up and I was trying to be compassionate because I felt guilty, although I was nuts about my new boyfriend and didn't care about my old boyfriend at all in that way. My new boyfriend didn't like me talking to my ex though so I put a stop to it.
However, one day I was very mad at my new bf because he had gone to Disneyland with his kid and his kid's mom. I felt very threatened and jealous about it all day and it really bugged me. So I went over to my ex-bf's house, and as a rebound type thing I got very drunk and we came close to having sex. But before we did I took off really fast because I knew I wanted my new bf, not the old one. I was really scared though cause my new bf had told me he can never go back with a girl if she had had sex with another guy after him, so I was terrified of losing him. So I called him when I got home, and he asked if anything had happened between me and my ex, even kissing, and I was so scared of losing him that I lied and said no. I know this was wrong but I was drunk and not thinking clearly, and knew I couldn't handle it at that moment if he told me to fu*$ off.
The next day though he found out from my ex all that had happened. Needless to say, since then he hasn't trusted me worth sh*%. He harasses me with questions about my past, wants every sexual detail, wants to know if I enjoyed the sex, why I had sex with these men, etc. It's been enough to drive anyone insane. But I love him, so I put up with it. It reminds me a lot of what goes on between you and your bf. My bf also tells me it will make things a lot easier if I just come clean instead of having him find out later, etc. But I get so scared of losing him, so scared of his reaction, or that he's going to misunderstand, that it almost paralyzes me. I feel its better to lie than to have him misunderstand, because I know that he is the one I love and the one I want. Even if other things sound bad, it is only an illusion. The only thing that matters is who I chose in the end.
I think you are the one your boyfriend loves, or else he wouldn't be putting up with it. I'm sure it's hard being in your shoes, but it's very hard to be in your boyfriend's shoes too. I know because I'm there. If he didn't love you, he wouldn't stand for all this, and he'd run back to his ex. I'm sure whatever happened was in a moment of confusion, and didn't have anything to do with whatever feelings for his ex. If he loved her and not you, why isn't he with her now? He chose you for a reason.
The main test is if he's willing to cut off contact with her. If you ask him nicely, he should respect your feelings and do it. I have cut off contact with all men from my past for my boyfriend. And once he has done this, in time you will come to forgive him and trust him again.