Trust issues because of both of our pasts

I have been with my girlfriend for 10 months now and I am having trouble with insecurity. My relationships before her ended with the girl cheating on me twice. When my current girlfriend and I met, we discussed our pasts and she said that on her last field work summer (right before we met) she cheated on her then boyfriend of 4 years. She said that she knows what she did was wrong but it also helped he realize it was really over. It was going in a downward spiral for 2 years at that point. I am glad she was open and honest but now whenever other men talk to her and I see her being friendly it upsets me, and this summer she went to the same field work trip. She has told me many times that the last time was different and she would never do that to me but it only makes me feel better for a short amount of time. We love each other very much and she is the jealous type as well, but I am afraid that I might never get over this insecurity. I feel guilty as well because even though she is just as jealous as I am, she volunteers a lot of personal information, like the fact that she cheated before, passwords so I could check something for her over the phone. She seems to share the jealousy but not the trust issues. She says it might be because she was never cheated on to her knowledge and can sometimes be too trusting of people.

She never confessed to him what she did, and tells me that she feels more guilty about what it's doing to me now than what it could do to him if she told him.
Don't people who get away with things like that and never confess eventually repeat? If they can get over it and not have to be punished for it, couldn't that mean they might feel more comfortable doing it again?
How do you get over being cheated on in the past and then finding out that this amazing woman also did that to someone else not too long before dating you? How can you move past the paranoia about her dancing with other men, when dancing is what led to her cheating that night on her ex?

It's not always true that they will cheat again, however, if she cheated on this guy just before she met you and then you got with her and you get worried now--why did you get in the relationship to begin with?

I can understand the jealousy part, but it seems like you both have issues going on on both sides. Dancing is just a symbol of the cheating, jealousy--not the actual thing. It could be baseball card collecting that would set you off.

Can you talk to her, a honest, goodness one on one conversation about your hopes and fears?

Actually thanks to the advice I've gotten on this site from people and their commentary, I got the gerbils in my head running again and was able yesterday to sit down and talk to her about it all very calmly and clearly. It went very well, not anything like I expected. She told me that she could never do that to me, that it hurts to even think about doing it and she understands why I have my insecurities but to think about it. If she was gonna do something like that again why open up and be honest about the story to begin with? Why would she give me access to all her personal info if she was going to betray me? And that she appreciates my efforts to fix my insecurity and own up to it rather than try to blame her for it.

And to answer your question, I gave her a chance because I had confidence she would not do it to me in the beginning. I think that confidence came from the very good job I had at the time. Money makes you feel very good about yourself and when I had to give it up to keep my grades up, I think some of my self esteem went with it. She assures me that she doesn't care about money and respects me for my decision for the sake of school. But I think that really wounded my self esteem.