I consider myself great at my job. At least compared to my coworkers, simply by being willing to actually do my job and not just sit on my ass all day collecting a pay check.
I was great at school until senioritis struck me during sophomore year. I could ace just about anything without studying, simply stopped doing most of the homework. I was also a great concert saxophonist through tenth grade, when I gave it up due to schedule constraints. I kinda wish now I had done bass in the orchestra. I'm (causally) trying to learn bass guitar now, and playing a stringed instrument after so many years of a wind instrument is making me question having called myself a musician. What is this more than one note at a time nonsense? It doesn't have a fixed tuning? Crazy talk!
I'm not nearly as athletic as I was in my younger days, but I used to be naturally good at most sports (basketball was a notable exception to this), and I was actually a truly great goalkeeper in soccer, my main sport. Really the thing I was or probably ever will be the greatest at. The one thing I'd consider myself to have truly mastered. Great enough at it I'm still pretty pissed I didn't have the support from my parents to actually pursue it seriously, because I had a shot at a pro career. My future kids will NEVER be told "that's too big of a commitment".