Girlfriend scared she's a slag

Hi, I'm hoping for help understanding this one.
I've been with my girlfriend for over a year now, we've lived together for almost half of that time. It is very much a long-term thing.
However, she is very closed when it comes to anything in the bedroom. She is relatively open with her past experiences (3 other guys) and i know she has done much more advanced stuff with two of them then she has ever with me. It does hurt but i wish for her to only do things when she is ready, but that never seems to happen. We have discussed it, she says she wants to but she says she is to afraid that i will view her as a slag if ever she did something. This even includes initiating any form of sexual contact. I try to tell her that it is okay for us to do anything together as we love each other and that the fact she remains so distance leads to me questioning whether this is really a hidden reason why she doesn't want to be with me. for example, she used to send pictures to one of her flings, very revealing ones! however i previously asked for a picture of her smiling when i was away on business, and all i got was a miserable selfie. I don't understand how she can be so comfortable with previous flings but not with her long-term bf. I've ran out of ideas of how to explain that there is nothing she could do wrong and that i could only view her in such away if she cheated. I don't know how else to approach the subject. I know the immediate responses will be that she isn't attracted to me or maybe in the relationship for something other then closeness and intimacy. But i'm convinced it is neither of those things.

Apologies to experienced users, i'm sure you can tell this is a first post!