Your brain can change, that is neuroplasticity, but not without limit and not for free. A plasticity budget is how much genuinely new learning you can consolidate in a given window before the returns flatten.
The mistake is spending it on whatever is loudest. Urgent and novel feel the same to a tired brain, so the budget drains into things you will never need again, and the skill that would actually compound never gets its reps.
Treat it like any scarce budget: decide in advance what deserves it. The point is not to learn more. It is to spend your limited capacity to change on the few things that pay back for years.