If you really like that girl and you know that she's a virgin then you'll unconsciously want to "give her the maximum" and that's why you can't get it up. Maybe you don't feel nervous but deep inside you are! You like that girl, you don't want to hurt her and this unconsciously blocks you because if it's not up, you can't hurt her.
I used to read a lot of letters like this in some sex help site. The psychologist there used to answer that you should try petting without clothes, just discover each other's body and then only slowly try the "main act".