hmm. well you should sit down and write out a list of good and bad things about her, about the relationship. then write down good and bad things about being single. think about being single. will you miss her more than the relationship? or will you be upset at having split up and then get over it? i think the thing is, once you've split up with someone and got over them, when you get back together you have the knowledge that if you split up again, you can get over them again. you also have the worry in the back of your mind that you WILL split up again, and hence you try subconsciously to get less involved in the relationship, to save yourself that potential hurt. when you first fall in love with someone, you imagine you'll be together forever, and you don't worry about splitting up. once someone's broken your heart, it's very hard to trust them with it again, no matter how much you love them.
there's two things i can suggest you do: number one is talk to your girlfriend about it. number two is talk to yourself. perhaps if you find out what she wants, then maybe you'll find she wants the same thing as you and you can fall for her again knowing that you're not going to get hurt. or perhaps she feels the same as you, in which case you just need to follow your heart.
if you're worried about her having been with other people, don't. from a girl's pov, once you're in a relationship, you don't think about anyone else from the past unless you're not satisfied with the relationship you're in. i know people can't stop obsessing over ex's (myself included) but the thing i always think of is: they were x many years ago, they can't still love that person because they're loving someone x years younger, and the person will have changed, got older, got a boyfriend/girlfriend, etc etc.