Anyone who's spent time in musician communities will be intimately familiar with the phenomenon.
Anyone who's spent time in musician communities will also understand just how good you need to be if you're still doing it after two decades.
I was actually a pretty decent bassist, way back, when mullets were en vogue, but I also knew that I wasn't good enough to stand out from the crowd.
In actuality, there's a direct relationship between how good a person thinks they are and how good they actually are. Not inverse. The Dunning-Kruger effect is that the people at the low end of the scale tend to rate themselves as slightly better than they are, and people at the high end of the scale tend to rate themselves as slightly worse than they are. The best people know they're good, but they tend to think they're not the best. The worst people know they're bad, but they tend to think they're not the worst.
DK is the most abused pop psych in the world.