Insert the cock just a few inches. Pump away until she's wet and wanting more. Slowly insert all the way, then push lightly and hold it there for maybe a minute. For me and my girl, that seems to push the cervix out of the way, just enough that I can pump for my own pleasure without bumping the cervix and hearing her yelp. Give your girl confidence that you don't want to hurt her, and she will relax and you will discover the maximum safe depth and then you will learn to not exceed it.