The brain is an organ. Like any other organ, things can go wrong. It's becoming the consensus that mental illness is caused by imbalances of hormones and similar things. Dopamine and serotonin in particular. It's an invisible illness, though, and so sometimes it's hard for people to take it seriously. Parents who don't think of "clumsy kid" as a potential problem might just assume they'll grow into their limbs. That's what people thought clumsiness in childhood was for a while- uneven growth that would eventually normalize.
People normally find out they're mentally ill when they realize the people around them don't struggle like they do. When I realized that constant soreness and tiredness and sense of dread was a thing most people didn't have, I called in to make an appointment and started searching the internet. Also, it's not a stupid question. Personally, I'm a mix of B and C. I hope you realize that it isn't your fault, because I haven't gotten there yet.
For a site extremely focused on fixing bad thinking patterns, I've noticed a bizarre lack of discussion here. Considering the high correlation between intelligence and mental illness, you'd think it would be a bigger topic.
I personally suffer from Generalized Anxiety Disorder and a very tame panic disorder. Most of this is focused on financial and academic things, but I will also get panicky about social interaction, responsibilities, and things that happened in the past that seriously shouldn't bother me. I have an almost amusing response to anxiety that is basically my brain panicking and telling me to go hide under my desk.
I know lukeprog and Alicorn managed to fight off a good deal of their issues in this area and wrote up how, but I don't think enough has been done. They mostly dealt with depression. What about rational schizophrenics and phobics and bipolar people? It's difficult to find anxiety advice that goes beyond "do yoga while watching the sunrise!" Pop psych isn't very helpful. I think LessWrong could be. What's mental illness but a wrongness in the head?
Mental illness seems to be worse to intelligent people than your typical biases, honestly. Hiding under my desk is even less useful than, say, appealing to authority during an argument. At least the latter has the potential to be useful. I know it's limiting me, and starting cycles of avoidance, and so much more. And my mental illness isn't even that bad! Trying to be rational and successful when schizophrenic sounds like a Sisyphusian nightmare.
I'm not fighting my difficulties nearly well enough to feel qualified to author my own posts. Hearing from people who are managing is more likely to help. If nothing else, maybe a Rational Support Group would be a lot of fun.