Ah. That's a different problem. I find it helpful to read memoirs of people who have been successful in the field. Surely You're Joking, Mr. Feynman mentions a number of episodes of him feeling like a fraud or a failure, as a graduate student or junior faculty. Many other scientists and scholars report the same. Feel free to talk to grad students or faculty around you -- I would bet that most of them went through this. Most of the successful grad students I knew felt that way some of the time or most of the time. Feeling out of your depth in science (and I suspect most other competitive fields) isn't an indicator that you're doing badly -- it's an indicator that you're badly calibrated. It's a routine feeling that you should just ignore until it goes away. See also the term "impostor syndrome."