When it comes to sex, people think they have to be like robots.. making sure they last forever (men) and the woman gets off. Women have their own anxiety issues; from my experiences it's mostly aesthetic, but it's anxiety either way you cut it.
so .. how can you get over it? Don't try to be a robot. Sex is about having a good time and just letting go and enjoying the ride. Get out of your (other) head and just let things flow. If she doesn't get off or you cream fast.. big fucking deal. That just means you get to practice more
Also.. make sure you communicate to your chick. It's ok to let her know you're nervous.. but here's a key.. don't apologize if you don't think you were good. It's pretty much a turn-off. Focus on the positives and go for rounds 3,4,5,6....