Having is so painful ,why?

when me and my bf have it really hurts im 17 and hes 17 also and he is a little oversize so when we have i have a really deep pain and it makes me very uncomfortable when he goes in but when he comes out the pain immediaty goes away???????????????? i tried telling him to slow down but being that he has a big it hurts naturally.whats goingg on ?. he pushes in so far that it feels like he cant go anydeeper.pls help me what can it be?