I think it's hard to imagine how smart some people can be. Einstein would hang out with people a lot more talented than him in mathematics (Poincare, Von Neumann, Goedel) ... so it's easy to see how he would be humbled.
During my PhD, I thought I was the dumbest one in the group. Some of the mathematical feats of my colleagues would take me weeks to process. But they would grok it in minutes. Eventually you have to take a holistic view: There are the things you are capable of, there are things you might be able to do but are not interested in (For me, spending 8h/day reading theorem, lema, proof statements), things you will never achieve, ... and combinations of skills that only you have and those people don't. Your path to satisfaction and happiness is often not becoming those "expert" people. But it's still great to be challenged and stimulated by them.
During my PhD, I thought I was the dumbest one in the group. Some of the mathematical feats of my colleagues would take me weeks to process. But they would grok it in minutes. Eventually you have to take a holistic view: There are the things you are capable of, there are things you might be able to do but are not interested in (For me, spending 8h/day reading theorem, lema, proof statements), things you will never achieve, ... and combinations of skills that only you have and those people don't. Your path to satisfaction and happiness is often not becoming those "expert" people. But it's still great to be challenged and stimulated by them.