And my level of intelligence bars me from getting much better than a C.
I'm sure you're more than intelligent enough to get A's, some people just struggle with getting their ideas and knowledge through. Do you ever end up writing too much and not finishing an exam? I've done that before.
Anyway, C is usually regarded to be average ability, at least in England. I don't see why it's such a disaster, if you get C's your still doing better than almost half the 16/18 year olds in England...