I used to be JUST like this... although, I don't really have much advice to give you
(( It REALLY REALLY sucks. My ex-boyfriend was more endowed than my current boyfriend haha... and so sex used to be sooo painful. We tried lube, but I kept getting UTIs with lube and then I'd have to take antibiotics, which would cause yeast infections, then it would hurt all over again. It was a mess! I noticed that (obviously) with yeast infections sex was unbearably painful, so make sure you don't have one of those! Also, as caterpillar79 mentioned, I KNOW that I have a "tilted" uterus (Dr. told me when I had an ultrasound done of my ovaries)... which you could very well have! A friend of mine had one before she had a child and she said sex used to be VERY painful for her. I know that it causes me discomfort often, because not all positions are pleasant.
Those are just some thoughts I'm throwing around... I REALLY can relate to you, I would get SO upset and just cry after sex because I knew it wasn't supposed to feel that way. I have to be very careful during sex, I'm just too sensitive I suppose. And having a boyfriend who isn't as large (which he feels all self conscious about obviously, while I'm very happy about haha!) definitely has helped.
Hope that helps! Good luck!