Try doing it on a harder surface like the floor. Put some towels down and that way things aren't bouncing around as much maybe. Your cock looks good and hard so I am thinking that is not the issue. Also, I could be way off base because I am just going by the info you provided. Are you two doing this on the bed? Tell her to be still and once you mount her, then let nature take it's course.
I have a feeling you will do just fine.