What Causes Pain And Difficulty To Do Intercourse?

Hi,
I'm a 24 year old man and I've tried patiently for over a year to do that myself to my 20 year old girlfriend, but she seems to have much pain with it and quickly asks me to stop it cause she is tired and it is hurting too much. We are both tired of that and, although willing to do it, we cant seem to loose her virginity. I've looked into her vagina and I believe that she might have trouble with it because she has a VERY small hole in her hymen to start from...