this is happening with my current girlfriend as well. There were times where I just decided to stop because I did not like the feeling of me hurting her. My best advice for you is to work her up really good. Start off with some foreplay, head down town for a bit with your tongue and fingers, and use a good lube. When you first go to put it in her, maybe just bob your head in and out. Then inch in deeper and deeper as your progress. You should stop concentrating on if your hurting her because that could be the reason your not enjoying it. Try to focus on her and make her feel comfortable so she gets the "oh its gonna hurt" mentality out of her head. Talk dirty to her, make her feel good. Use your free hands to massage parts of her body. Trust me, it will take some time for her to get use to it. It has almost been a year for me and she still says she is sore the next day, and I am not that big.
Or if all else fails, have a few drinks before hand. Will make her loosen up and less tense and will make you last longer too! LOL. jk. I do not promote the use of alcohol or drugs in any matter. | 
