If I may add something, in line with several other comments here. First of all, as others said, your worries are normal: at a certain point in life, you have to realize that there are "better" people than you are. It's part of growing up, of becoming mature. Of course, as long as you think that you are the next Einstein, this is somehow stimulating - but sooner or later you will have to confront hard reality. I think it is a kind of syndrom that many "bright students" have to go through. I did too. Point is, if you are more than average interested in a subject, and you think a lot about it, chances are that up to college level, you will far outshine any of your co-students. So your only reference point is that you are "the best" in your immediate social environment, and that during years. Even if sometimes you aren't officially, you can always tell yourself the excuse that you have been unlucky on that occasion or something. That doesn't mean that you don't have to work, that doesn't mean that you won't get stuck,... but you compare and you see that others in your immediate social environment don't do really better. You have your chances to be "the best".
But at graduate/PhD level, suddenly you're mixed in with other "best guys" and you see that mileage can differ. That the world is a much bigger place than your classmates of high school. Worse, that you've made wrong choices in where to do what with whom. That the skills you found yourself so brilliant about, are, well, "not always in the market". So you finally realize that the next Nobel is NOT going to be yours. That it will even be terribly difficult to do SOMETHING immediately brilliant. At some point, you'll have to go through that bit of growing up (unless you ARE one of those happy few, but statistically, chances are much greater that you win the lottery...).
So what to do next ? Give it all up ? Throw it away ?
No ! Unless you did win the lottery, or are of rich descent, or will get married to Paris Hilton or something, you will have to do something terrible in your life: make a living!
And, at the same time that the world "got bigger" and you realized how much more smart guys and galls are out there, the world also got bigger concerning possible things to do. You wanted to be the smartest scientist, simply because, well, it's the only thing you could think of of doing in your life. But as your world view opens up, you'll see many other opportunities. You won't know what will there will be, you have to keep an open mind on what to do. It's good to have some dream (to keep motivated), but it is also good to let it go at a certain point, if you see that you can do something else.
And now comes the important thing: whatever you will want to do later, and whatever opportunities that will present, it is always more fun to do something that has something to do with stuff you like and you are good at, than "just anything". Scientific skills, mathematical skills are useful in society in a gazillion ways. Not only to "make science advance". In fact, if it were only to make science advance, science by itself wouldn't have much usage. Using science, using math, to DO something in the real world can be very rewarding. You might be surprised - well, *I* was - how much you can help in different places with just first or second year mathematics, thoroughly understood. You might be surprised how many people - even in important positions - aren't able to work out a mathematical problem related to their field of expertise, from beginning to end that you would expect to be an exam question in first or second year.
So there is A LOT OF ROOM for people having some good understanding of science and mathematics. You can do a lot of interesting things. You can be very useful (and be rewarded for it). Even if you are not Euler. Especially if you are not Euler, I'd say.
As a summary: in as much as you discover how many terribly smart people there are around and you're not in the competition for a Nobel or Field medal with them, at the same time you will discover how many more ignorant people there are that you will be able to help out (and are willing to pay you) with moderately good math / science problem solving skills.
At least that's my experience.