Painful sex??

Can someone help answer this question? I'm going to talk to my gynecologist about this soon, but every time I have sex it ends up becoming painful. I don't mean painful on the outside of my vagina because of friction or sores or anything, because if my boyfriend is really shallow, it feels great. But he can't even go all the way in without it hurting deep in there so bad to point that I can't handle it. I involuntarily jump back because of it. He doesn't even have a huge penis either: it's only about 6 inches, if that. I regularly use lube and I am a firm believer of foreplay. It's just no matter what I do, I can't have sex without being scared it's going to hurt. Does anyone have any suggestions as to why I'm like this, or solutions to fix it?