Aa, the reason I asked your age is because your question is one that a lot of young people ask. I see from your other responses that you are 20. I also see that you haven't really had the kind of communication with your girl that you want to have due to her shyness and discomfort. You mentioned that she has been with other guys but are you 100% sure she isn't a virgin or someone with very limited sexual experience? She just might be afraid. If she was raised in a household where she was told sex is wrong, she may be suffering from mixed feelings about this whole thing. Maybe she wants to have sex with you but is afraid once she does, you won't want her anymore. It really could be any number of things. This really is all about her and what is going on with her, not you. At your age, I can understand your confusion and your doubting yourself. But trust me, it is all about her. She knows what you want and she is uncomfortable.
By the way, other people here were telling you to start some other hobbies or do other things instead of dwelling on your issues. It isn't a light switch situation, it really has everything to do with getting out and spending time with your friends or doing something else that you enjoy besides driving yourself nuts over what is going on in your girlfriends head. We can't do anything about the things we don't have control over. AND, you do not have control over her feelings and thoughts. Her hesitancy or shyness to discuss this aspect of your relationship is a HUGE problem. Not being able to communicate your feelings to your partner and work on a solution, will absolutely doom a relationship. It leaves too much room for speculation -- which is what you are doing. When we sit down, and try to figure something out that is going on with someone else without any explanations, we just get nowhere except to really drive ourselves nuts. So, you need to start refocusing your attention on the things in life (other than your girlfriend) that you enjoy. Get your butt out and hang with friends, go visit your family, go volunteer your time to an organization that you believe in (ex, volunteer at an animal rescue group or be a "big brother" to a boy who doesn't have a father or brother to do things with).