As far as caring what others think about you, this is instinctive, and can't be simply wished or willed away. What I would do instead is to try and focus on the opinions of you that actually matter, such as those held by your teachers, your football coach, and other such authority figures. You also want to try and surround yourself with people you care about, and who also care about you, and focus on their opinions. And when you get criticism, let it motivate you to do better, instead of just dwelling on it.
As far as feeling stupid, know that intelligence is not scientific, which makes stupidity a subjective term. To see what I mean, check out "Myth of Intelligence" by Patrick Winn.