Hardknock

I have the same problem. I'm 7 inches and more times than not, I'm hitting my wife's cervix every time we have sex and she's on the bottom. Like Shani and La Petite said above, the only way we've been able to prevent the pain is for her to be on top so she can control how deep I go. Other than that, the only thing that works is for me when doing missionary, doggie or anything with me on top is to control how deep I go in. What I do is put only the head of my penis inside her and thrust in and out with just the head and tease her that way. She does call it being a tease by the way but what it actually does is get her more lubricated and excited and she always ends up begging for more. When I finally slide all the way in it doesn't hurt her as much since she's totally ready for it at that point.

Give that a try. Just experiment and see what works.