Smoo, when I was younger I had a few girls that it would hurt when they were on top because they would get going and lose controll (or I would would just grab thier hips and take over) and my penis would go to deep. If this is the problem, doggie should work well is she lays flat and just sticks her butt in air a little, should be able to knock an inch or more off. If it is because you are too thick, get some sex lube. It will help a lot (after the first time I used it, I wanted to buy stock in the shit) and just make sure you are not cutting the foreplay too short. And, there is the posiblitiy that you both just need a lot of practice, FUN!!!