You can do a hell of a lot better than this girlfriend, trust me!
Sex is the ultimate dance. Sure, it helps if you know the steps, but the bottom line is what is going on between you two right then. If she's not getting what she wants it's not because you are doing anything wrong, it's because you two aren't communicating effectively (probably more outside the bedroom than inside.)
She's using your inexperience as an excuse for her own indecision. You mention that she is torn between three separate men -- you and two other guys! Jeez, if that doesn't put a damper on your sex life, what will? Talk about performance anxiety.
Hey, you are not performing, you are loving. If she's judging, that's not love, that's competitive screwing. It shouldn't be you against the world of men out there. It should be about you and her -- that's it.
So, I would really recommend you re-evaluate this abusive situation. I think she is abusing you through her own indecision, and causing you to question the most personal of interactions.