I have had this problem with a regular once, problem solved by stopping masturbation at least two weeks before a punt. If that don't work then kamagra my friend.
Maybe you like her and have performance anxiety, (trying your best to please her). This also happened to me once i was hard but could not for life of me cum, i nearly tore the skin of my dick trying to cum on her with my hand (successful but i felt like shit, i basically paid for a wank).