Why can't i get fully erect anymore?

Im 18, and i just recently stopped masturbating and watching because of this problem. I have a gf now and when we tried to have ,but i wasn't hard enough. I mean shes pretty and everything and her body is like oohh!! I wanted to be hard, but it just would not go fully. I don't know if i was nervous or what, i mean it was our first time having but i think since i masturbated so much and watched my body is used to only getting hard that way. When im fully hard im almost 7, but now i can only get almost 6. What the efff, please someone help, pleeeee.