I simply don’t believe this is true for everyone. It’s certainly not for me.
Math is the only area where I’ve ever deliberately studied, and saw no improvement, even with huge number of hours using many different methods with full dedication on my part. I can memorize many things, and make sense of the concepts just fine, but when it comes time to actually doing the problems, unless they are completely identical I am lost.
Furthermore, when I do manage to memorize enough to determine what to do, the moment I learn something in the math arena but not related to what I was studying before the previous learning is utterly extinguished. What I mean is that, say, I learn how to handle factoring. I have it down from an operational standpoint (I always have understood the concept of factoring just fine). The moment I learn for instance how to do some geometry, factoring is wiped. It’s just gone, never to return.
Whatever it is in most people’s brains that allows them to remember more than one math idea at a time (from a working-out-the-problem standpoint) is just not present in mine. I suspect it’s because so much of my mind is devoted to languages and data analysis at a high level, there just isn’t space for anything else like this.
In high school, a teacher was astounded that some algebra techniques that I’d seemingly mastered just a few weeks before I could no longer remember even the first thing about. Sure, I remembered doing them. But I had no idea how to perform them any longer. This is the case with anything in the math realm, no matter how much time I devote to it.
With great effort, I can memorize enough for one test and do well on it, but a week later, all is forgotten.