My guess is you are nervous and worried about your performance. I am not an expert but everything points to that. The only time you have an issue is when it gets down to sex. First of all, I would be open and talk with your girl about what is going on. One idea is when you are kissing and hard, ask her to stroke your penis for you. If you stay hard and she makes you ejaculate, that is cool. You can please her in other ways. Regardless, take it very slow, don't rush into anything. Go slow, and if this doesn't work for her, find a sweet girl who is patient. You will be fine.