I really feel for you. Can't imagine what has kept you away from relationships for all of your life, but the important thing is that you are now willing to settle into one. It is a delicate thing you are trying to handle here, because a woman's sexuality is so intrinsically tied to her emotions. This can have it's positive and negatives for and against you. On the negative side, she will be waiting for you to straighten the situation out, or else. But on the positive, if you are honest with her and show her sensitivity, NOT weakness, she will sympathize with you and become more patient. In other words, sensitivity is being able to explain to her how much you want to please her, and how much you care about her as a person, but at the same time showing her that you have strength of character and conviction, and that you will try to overcome your hurdles.
In the meantime, two things you can do is one, learn all you can about the female body and how it functions. If you have spent a lifetime looking at p 0 r n, you most likely have adopted some totally erroneous ideas about women's sexuality. There are a number of good books at bookstores dealing with this topic. One thing to remember is the ~clitoris~!!! Women rarely cllmax from just having penetration without first having a nice clitoral cllmax. This is brought on by your attentive and loving attention. Also remember that although a penis is wonderful to have after a clitoral cllmax, a woman can be very satisfied with just the clitoris if done right. By concentrating attention on her cllmax this way, it will take the edge off the anxiety and rush to get your end of the deal worked out.
You could really use some professional help. You need to do some retraining, and with the performance stress that you mentioned this will be a difficult feat. Having a therapist that can guide and assist you along the way can make things a lot easier for you. If your girlfriend is a truly loving and understanding person, she might just feel good about the fact that you are taking things seriously enouh to really do something to solve the problem.
Sometimes women just need to see that you are being sincere. A positive and pro-active attitude will work in your favor.