What this guide covers
Why most improvement efforts don't work
Tactics puzzles, opening study, YouTube grandmaster games — none of it is wrong, but none of it is targeted. Improvement comes from fixing the specific thing costing results in your actual games, not from broad knowledge accumulation.
The mistake you make in 15 games matters more than the blunder you made once in a spectacular loss. Targeted work on the habit that repeats is more useful than reviewing the game you feel most bad about.