I had the same problem when I first met my now wife. I was and still am extremely attracted to her but the first time we were to have sex my guy showed up for work but didn't go the whole shift so to speak. It actually became an even bigger problem because the next time I was even more nervous and it happened again and it snowballed from there. I went to the doctor, got a physical and everything was fine. My doctor being cool as hell gave me some free Levitra and a prescription for it. That stuff worked wonders and helped me to get over the hump so to speak. I am not saying that it never happens anymore because it does from time to time, mainly when I am tired or stressed so i keep the pills on hand just in case. The biggest thing is to communicate to your girl that it's not her, it's just a mental thing. You HAVE to communicate with her. Good luck with it.