You two are incompatible, plain and simple.
You found yourself in a place of judgment, with her judging your abilities in bed and you trying to measure up to her expectations. You became a dancing monkey, wondering what is wrong with you. Guess what? There is NOTHING wrong with you. You both weren't compatible, that's all.
The only thing wrong is that you are an AFC (as most of us here used to be) and you are learning how to be a man in control of his own destiny and his relationships.
Your incompatibility was a guaranteed fail. It couldn't have been otherwise. You should have dumped her when you detected problems.
Guys, always remember that when men get dumped (by someone they actually care about), it is a LOT harder on them than it is on a woman. The woman has a support system, a web of friends and family that she can cry to and get it out of her system. Plus, she gets constant attention from men. That equals an ability to get over you quickly.
We men, by and large, are loners and we process things internally. Therefore we tend to ruminate and get stuck in thought loops. We wonder what is wrong with us, instead of learning and making corrections and moving forward.
Do no allow yourself to get dumped, ever! Please read and sign this inter-office memo:
When a man senses that he is about to get dumped, he will, 100% of the time, get dumped. There is no deviation from this maxim. Zero.
When you sense impending dumpage, you MUST dump her unceremoniously. Tell her that you have decided it's not working out and that you want to stop seeing her. Be ruthless. Believe me, what you don't understand is that long-term, she can take it much more easily than you, as a man, can.
By igniting a nuclear dump, you will completely flip the script and shock her beyond belief. This will knock her right off the lofty pedastal that she is looking down at you from. After you dump her, if there is a possibility of things working out, she WILL go into begging mode (later in the process) and then YOU will decide if it is to be or not. If you don't hear from her again, then there was no chance under heaven that it would have worked out.
Either way you win. With the knowledge that we have, none of us should ever be in a position to get dumped by a girlfriend. There is a place for love, to be sure, but there is something that eclipses the importance of love. That is being a man, controlling the frame in the relationhip, and managing it.
YOU must always be the manager of the relationship, not her. In all things, the man is the giver and the woman is the receiver. This applies sexually as well as in all other areas of a relationship. It is one of the paradoxes of life. Her proper place is a position where she is lead by you. When you have contructed that dynamic, she will naturally give herself over completely to you.
OP, welcome to the world of enlightenment. It hurts for a while, but better days are coming, wherein you will not be working to please her, but rather women will be working to please you.