Too big?

Breanna • Hello! 🌸💕
My boyfriend is huge. Well, I think he is. I have no prior sexual experience and I have nothing to compare him to, but he says he is much bigger then average . Around 10 - 11 inches. Longer then both my hands side to side (in a fist) ! 
Anyway, we have tried it from behind (in my vagina) and he hits the top of me on the inside! It is very uncomfortable, and I lose all my arousel immediately. He must be too long, I'm not sure. Is this possible? What can I do?