If you're in grad school taking qualifying exams, memorizing theorems is required. There's just too much stuff to digest. Undergrads and younger students have

If you're in grad school taking qualifying exams, memorizing theorems is required. There's just too much stuff to digest. Undergrads and younger students have ample time to learn mathematics with serious depth. This is not true for the average math grad student. The comfort you had in learning mathematics as a youngling just instantly disappears when you get to grad school. This is painful, but you have to learn to accept that you won't be competent at all the mathematics you are exposed to. Some very fine mathematicians experience this.