"I feel like this is some kind of rule of improvement: in order to get better, you must feel uncomfortable. And if you get to a point where you feel comfortable, you need to force yourself out of that situation." https://illiteracyhasdownsides.com/2020/06/13/how-to-get-worse-at-starcraft-ii/
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This is exactly what learning high level mathematics is like.
Just when you feel comfortable with an idea, or a method, you find it has all been an approximation, and then you are introduced to a another more complex system.