I am 21 years old and have been dating a girl for about 7 months. I like her a lot and in the beginning when we started fooling around I experienced the inability to keep an erection before penetration. I was fine for foreplay and everything but when I tried to put it in it went soft. I stopped trying after about five or six failed attempts and did everything with her except sex. I have only been able to have sex with her like 6 times over the 7 months, I am not saying I try a lot but when I do I have experienced over 75% failure rate. I do not know what is wrong, when I masturbate I can keep an erection but if I stop touching myself or if she stops stimulating me it goes away pretty fast, like within 30 second. I do not notice that many morning erections and when I do i feel like they could be harder. I am healthy, and in good shape and exercise regularly. I have to say I do masturbate on top of what I do with her, so I do not know if this problem may be caused by over ejaculation. I saw a urologist and he said it was performance anxiety but how do I get over this? It has been several months and I want my sex life to be good with this girl otherwise I am just going to be depressed. Someone help me.