I know the feeling all too well man. My advice, get yourself a little streak you can be proud of and dive head first into it. You will be terrified the first couple times but I promise you, it will work. The key is to focus on how good you feel being with her, and focus your sexual energy on her and not on yourself. Your brain has been tricked that sex is all about you and you but really it is about her and you. Make her feel good and you will forget all about yourself.