well, there's no way anyone online can really tell you exactly what it is...it sounds like it mght be better if you had another chat with your doctor, though.
i know you don't think it's lack of lubrication, but adding extra lube can't hurt and it might help a lot - i would say give it a shot, astroglide is always fun. if you have vaginismus or some other type of muscle tension, making things more slippery is likely to help at least a little bit.
vaginismus doesn't need to be "caused" by anything, it can just happen. talk to your doc more about this, maybe?
the other thing is to really make sure you're fully aroused, that really helps a lot.
Boys and girls in America have such a sad time together; sophistication demands that they submit to sex immediately without proper preliminary talk. Not courting talk - real straight talk about souls - for life is holy and every moment is precious. I heard the Denver and Rio Grande locamotive howling off in the mountains. I wanted to pursue my star further. -Kerouac