It's all a matter of sensuality. If you can manage to get yourself caught up in the sensation of the experience, then you shouldn't have a problem. You're probably so worried about something OTHER than enjoying yourself, that your body's not properly prepped to continue with the deed. (I'm assuming that the assurance of an equally pleasurable experience for your girlfriend is included in that worry.)
If all else fails, extend foreplay. Good luck, champ.
