Look at it this way. Your abilities/worth/knowledge/whatever are multidimensional and can be mapped on a "Radar Chart"(https://en.wikipedia.org/wiki/Radar_chart). They can also be classified as "Inherent" or "Incidental/Accidental". Thus there can be no single "smartness" index defining a person.

Thus for example, i am "un-smart" on both Web Dev and Statistics/Probability axes. The former is Incidental and easily fixed in a couple of months of work but the later is Inherent and needs a lot more time for assimilation. Thus the same word "un-smart" actually conveys two different meanings in the two different axes and one should not conflate the two.

Same here buddy. I’m 30 and self teaching myself 2nd/3rd year undergrad math courses that I never learned since I wasn’t smart enough to pass the prereqs. I suppose like you I’m also considered competent at my job, but it’s not lost on me that people younger than me mastered this material a decade ago and are publishing cutting edge research. That’s just how it goes.

This past year I’ve had the misfortune of having an immediate family member suffer from a grave illness. It made me realize feeling bad about myself was just narcissism, and I need to continue to be my best but also spend less time feeling sorry for myself.

Best of luck to you in 2020 :)