As mentioned, my new year's goals include going through GM Smirnov's "Grandmaster's Positional Understanding." One lesson in particular has really opened my eyes. It's not even about chess per se, but rather about training in general. Essentially, there's a world of difference between knowing something and knowing something, and we need to train ourselves to get from the one to the other.
He specifically included this because some of his lessons seem simple or even obvious. "Develop my pieces? Thanks, Igor, I've known this since I was rated 800." Smirnov's point, though, is that many players (and not just beginners) know they should develop their pieces, yet they frequently play games and fall behind in development for whatever reason. They chase material, they worry about pawn structure, they try to attack, etc etc.
They might "know" about developing their pieces, but it is not their first thought when they look at an opening position. Or, alternatively, they don't know how to do it: they might develop their pieces to poor squares, forcing them to move them again, falling behind in time. Basically, until "development" is their first thought and they do it easily and effortlessly in any given position, they don't know it.
This lesson struck a chord with me. I don't remember even reading it the first time I did GMPU, honestly. I'm not going to reproduce the whole lesson here, for obvious reasons, but I am going to discuss the general themes, which resonate with much of my earlier thoughts and writings on chess.