How can I be more clear-headed?

I'm not dumb. I'm a national merit scholar, a straight-A student and got a 2280 on the SAT. However, most of the time I feel like I'm functioning on a lower level than many people around me. My mind feels foggy and I rarely feel like I have a good grasp on what's going on around me. I can't speak without mixing up my words. I can't play games without getting the rules confused. It's embarrasing, since I know people expect more of me and are often disappointed in my actual performance. Does anyone else feel like this? What can I do to become more clear-headed?