I would say that grad school took me from the knowledge that I was one of the best of the math students in my year, to knowing that I was a mediocre mathematician. So although I came out of it with better skills in math, I had a worse self-confidence in that area. This is probably a common experience, and not strange, because as a student you're working with small, self-contained problems. By contrast, as a grad student you're working with large, open-ended problems that take years to solve. So it sounds like my experience was more like cahn’s. I do, however, think that I am a pretty good university math teacher.