I'm so sorry to hear about your difficulties. It's really hard to say whether you might have a "sick brain," as you put it. It seems possible that you could have attention deficit disorder or learning disabilities. Ideally, those kinds of issues should always be recognized and addressed in childhood, but sometimes a child with attention deficit disorder or learning disabilities will slip through the educational system without ever being diagnosed. Other possible explanations for your situation include a lack of study skills and low academic aptitude -- neither of which is a disorder. It sounds like you need an evaluation to help you figure out exactly what it is that you are up against. Good luck.
What about your fitness level? Do you run or do any sports every week?
What about your emotional life? Is everything good around you/family friends etc?