I'm Having Trouble Applying the Idea of Deliberate Practice ?

Chatted with some other artist about this idea (learning overall), practicing by focusing on one aspect at a time, setting a measurable goal, using errors as feedback on what to practice/study further n repeat (plus some working in the goldilock zone and other things).

It makes sense in writing but I'm really having trouble actually applying it.

Right now I'm focused on still lifes and perspective and since I feel I struggle the most with getting the direction of planes and ellipsis I decided to narrow in on that for a kinda trail run. Trying to match the angles of plane like objects irl and some from 3D program, measuring how they compare and why maybe it's off. Draw another with the new info in mind etc

But I couldn't actually figure out what was wrong, just that an error was there. About three pages in and still couldn't find a pattern, they were just wrong.

Does anyone have any advice on this?