Hrm...sounds like she has some trust/intimacy/commitment issues. I'm usually quite puzzled myself when I hear someone make this excuse to stop seeing one another - that it's "too good to be true." It seem so...dramatic, and usually when people go into relationships with an attitude like that, it usually means that they're signing up for a self-fulfilling prophecy. Even when things continue to go great, somehow they will find a way to find something wrong. I'm not sure why people sabotage themselves like this...to me it either means that they either don't actually WANT to be in a relationship of any kind, or because they haven't let go of something in their past that they NEED to let go of in order to have a healthy, happy relationship.
A part of me also wonders if she has a maturity issue, as well.
I'm not sure how I would handle this...there never seems to be a "correct" way to do so. Quite honestly, I think she's put the ball in her court, whether she knows it or not - SHE has to be the one to take the leap of faith here, since you have already taken it. She has to decide if she wants to go ahead and risk getting hurt again, which is what we all sign up for when we get into a relationship with someone...if you don't take that risk, then you're only half-arsing the relationship! I don't know about you, but I can't love people in "slices."
This sounds like something she seriously needs to work out, particularly if she doesn't like seeing you with other women friends. I often feel like pursuing someone who has already said that you should stop dating will continue to push them away. Granted, some people say such things because they want to be pursued, but in those cases, those are not people that I would personally want to be with - they're games and manipulations...a bad combo for a good relationship, in my opinion.