Dunning-Kruger
Wikipedia explains the Dunning–Kruger effect:
Across four studies, the authors found that participants scoring in the bottom quartile on tests of humor, grammar, and logic grossly overestimated their test performance and ability. Although test scores put them in the 12th percentile, they estimated themselves to be in the 62nd.
So, as you learn more, you tend to become less confident in your abilities. This is not great… but I do believe it leads to a good side-effect: as you learn more, you end up with more responsibility, and you tend to be more cautious. Or… you suffer because those under you get promoted because they believe (and convince others) they are more competent.