Doctor, Im Unable To Penetrate My Girl Friend's Vagina, Even

doctor, im unable to penetrate my girl friend's vagina, even after having proper erection. im very much comfortable with my last girl friend, but her vagina was loose, i felt smooth entering. we had a baby too.
But with this girl, im unable to penetrate itself. She feels severe pain, keeps her thigh tight erect and move behind for my every push. this embarrass me very badly. Is this her problem r mine. how to comeover it. please help.