I never learned how to put forth effort, because I didn't need to do so until after I graduated high school.

I got into recurring suboptimal ruts, sometimes due to outside forces, sometimes due to me not being agenty enough, that eroded my conscientiousness to the point that I'm quite terrified about my power (or lack there of) to get back to the level of ability I had at 12-14.

I suppose, if I had to give my younger self advice in the form of a sound-byte, it'd be something like: "If you aren't--maybe at least monthly--frustrated, or making less progress than you'd like, you aren't really trying; you're winning at easy mode, and Hard Mode is likely to capture you unprepared. Of course, zero progress is bad, too, so pick your battles accordingly."

Also, even if you're on a reasonable difficulty setting, it pays to look ahead and make sure you aren't missing anything important. My high school calculus teacher missed some notational standards in spite of grasping the math, and her first college-level course started off painful for it; I completely missed the cross and dot products in spite of an impressive math and physics High school transcript, and it turns out those matter a good deal (and at the time, the internet was very unsympathetic when I tried researching them).