There are very different learning types. I for one am very bad at memorizing facts and forget them quite fast after not using them any more, no matter at which age I learned them first. Concerning mental arithmetic, I was quite good in 5th and 6th grade, but now I am painstakingly slow even with simple multiplications like 7•8 ("Seven times eight was something like fifty-six, right? Let's check, so seven times seven was forty-nine, forty-nine plus seven… yeah, fifty six"). I just don't typically have to solve such problems at this time in my life (the closest I've come to needing arithmetic in the last few months was when I was studying Galois theory). But at least I am confident, that I can train myself, should the need arise.

The most frustrating experience with memorizations in school were poems for me, because it always took so much effort and even while I was slowly mastering it I knew it was all for naught, because in a few weeks everything would be gone again, even with those that I actually liked.