Extreme pain during sex

I've had this problem for a while now but sometimes during sex it hurts so bad. The last time we did it I almost started crying. It's scary every time because I don't know if It's going to hurt or not. I hate having to push through the pain just to make my boyfriend happy, but I would do anything for him. I just wish I knew what was wrong so I could fix it. Can anyone tell me why this is happening