Painful sex!?!?!

So when my boyfriend and I have sex and in certain positions i can feel him hit the end of my tunnel if you know what I mean ? He goes too deep and I feel as if he's hitting the beginning of my cervix or just the end of my vagina. Does anyone know anyways to prevent that from happening or hurting ? Because its always my favorite positions it happens in and then I have to push him back because of the pain I get. It happens almost every time we have sex depending on the position and angle we are having sex in. Is this preventable in anyway ?