How many nba players do you think spent years working on layups and nothing else? The push themselves. They work on what they're not good at yet, and in doing that, improve at the things they already are.

Spending years going to classes that do nothing but teach you things you have already mastered is not practice. It's torture. To improve retention and mastery you keep using the skills you've attained to greater effect. The compounding is what matters. Move too slowly and people shut down, tune out, and ignore the material. That's when material is forgotten.