Uncomfortable about hidden past relationship

Although I keep telling myself that it's not a big deal, but somehow I keep feeling a little uncomfortable. My fiance had told me about most of his past relationships, except one. He said he thought I'd take it the wrong way because I also know who the girl is. They're still good friends. He said he tried telling me but he didn't want me to take it the wrong way, and that he didn't want to lose me. He said he was really sorry for not being able to tell me earlier. He's told me that there's nothing else that's left which he hasn't told me.

According to him, it didn't last a very long time. After a few months they realized things weren't going to work out. I think it's perfectly normal for people to be in relationships, but what I find hurtful is the fact that he hid it from me. It seems to me that they're still very good friends...and somehow I feel uncomfortable about it...mostly because of my own insecurities.

I've somewhat become a little possessive about people I care about. I've developed this jealousy and fear of being hurt because of a past relationship. I was being cheated on by my boyfriend for a few months until I found out about it. It was the first time that had happened to me, and you can imagine how shocking and upsetting it must've been. Since then, I've sort of been hesitant in trusting people. There have also been other incidents in life where people had lied to me and I felt really hurt.

My fiance is a great guy. We've been really happy together, but since the last few days, I've started feeling like it's all too good to be true. I keep getting negative feelings and I'm hating it.

Please tell me how I can overcome this jealousy/fear. I don't want to feel scared... like everyone's out to get me. I want to be able to trust people completely.