Help!!!!!! Firstt time Problems!!!!!!!!!!!!!!!!!!?

I was with my gf for about an hour and we were grinding up against each other and doing other stuff and i was realy enjoying myself as she could feel. Then we decided to take the next step and have . I had been dreaming about this all my life and then all of a sudden i just couldn't get hard again. Thi happened yesterday and its all i can tink about, i was so embarred i'm scared it will happen again and now thinks ive gone funny with her. We have been seeing each other a month and nearly every other time i would was dying for something to happen and i would have been able to perform. My gf said that i shouldn't worry about it but thats i can think about. I never expected this to happen to me, i always expected it t be the other way round. Like not lasting long. I just want to know if this has happened to anyone and what do they do about it. I have always been able to get hard, even when i don't want to but ever since that nite i have been able to get completely hard.