So look. I've been in a situation similar to yours but a little more fucked up.

At the tail end of college my wife was in a serious relationship with a guy that was a real piece of work. One day she found out that he was fooling around on her and was signed up for all sorts of adult dating/swinging sites to try and get some side action. She went along with it because she wanted to try and salvage the relationship and agreed to start swinging, going to orgies, shit like that. They invited other people into the bedroom or he would just go out and screw other women and she hated it but kept it up just to try and please him. Eventually she realized how terrible this all was, that she didn't enjoy it, and that she was basically being turned into his little whore. When she stopped doing this, he ended the relationship and it destroyed her mentally to know that she did all of this for him and he threw her away like it was nothing. That's all another story in itself and not my story to tell, but the background is necessary.

She regrets every bit of it now. She's often ashamed that it ever happened and that she let it happen. She doesn't even know how many people she's slept with. There are times when we will be intimate and she just starts feeling ashamed by it all and has to stop. There will be periods of time where she doesn't want to be intimate at all because she obsesses over feelings of guilt. Yes, it really sucks to be mid thrust and have to stop and comfort your wife who is sobbing uncontrollably because she simultaneously feels ashamed of herself and guilty for having it affect our relationship. But we work it out. I give her some space when she needs it, and eventually we get back to the awesome sex. (for the record, yes, she is in therapy and has been for a while for some mental health issues.)

So, could it be the past relationships? Maybe. Could it be you not being good enough? Maybe. It could be any number of things, but the important point I wanted to get across with this whole thing is that maybe it has absolutely nothing to do with you specifically and if she isn't willing to work on your sex life and find mutual satisfaction then you don't have one of the major building blocks of a good relationship. Stop worrying about all the other penises she's seen and worry about what you do with yours. Decide whether or not you want to try and work through this and your girlfriend needs to do the same. If one of you isn't willing, there's not much point to trying to go on, honestly.