How to make "it" work?

My fiance has told me that sometimes my dick feels too big and it hurts her when we're having sex. Sometimes I've had to stop because she is in a lot of pain. At times it makes me think that our relationship is not going to work based on that BUT there are times when we may have been drinking or both are extremely horny and we caught up in the heat of the moment so it isn't painful then. We can both admit the sex is good on both ends, but I know it can be better if we could come to some sort of an agreement when it comes to doing the "do". The problem is more often than not, the sex is painful for her. Seeing that we're going to get married and our sex life is already suffering in some areas, is this an indication of things to come. Help.