My girlfriend has been pushing our relationship to be more physical?

I don't think that it is a bad thing. I am just really apprehensive and nervous about my performance. I don't think that i will last very long and don't want to disappoint her. I really like her and there have been times when I have punked out because Im nervous. She is definately more experienced ually than I. I ve talked to her about this and she says that it is fine but I really like her and don't want to have a big letdown. Any advice?