Hey WTT:
I'd second Starkey's suggestion that you consider a therapist. You don't need to be depressed to see one (and it doesn't sound like you are depressed). I'm constantly down on myself, to the point where I have trouble accepting compliments from people I otherwise trust. When I saw a therapist a few years ago, she provided a much-needed outside perspective on my interactions with others, and often pointed out positive things about my personality and intellect that I had never considered.
There are lots of ways to feel smarter -- practice helps a lot. I play a game called
Set every day, and it's gradually improving my pattern recognition skills. The folks you're jealous of aren't necessarily smarter than you by nature (though it's possible they are); they may just have skills that let them excel academically. And on a less charitable note, they may have simply realized how to "work the system" to get better grades. School isn't always about being smarter.
Good luck.