I know, "practice, practice, practice." But is that really what professional mathematicians do? Do you actually solve a differential equation every day, do a contour integral every day, do a SVD of a matrix, compute a sum of random variables, work a detailed vector calculus problem, a group theory problem, a differential geometry problem? That's just listing the subjects I once knew, and it seems as if that would take hours every single day. Plenty of people here seem to know a great many more mathematical subjects as well. How do you hold it all in your head at once?
I have written out many problems and saved them, and outlined books in great detail (Spivak's Calculus on Manifolds, Korevaar's Mathematical methods book, Linear Geometry by Rafael Artzy, a quantum mechanics text by Bransden and Joachaim, Wackerly's Statistics text, etc.)and try to reread the notes, but it still seems that the knowledge decays just as fast as I can add it or restore it. And a subject like differential equations! When working a single problem takes 15-30 minutes, I don't understand how anyone gets real mastery of that in the first place!
Is it just a matter of my lacking sufficient talent? Is hours of review every day simply a normal part of being a mathematician that nobody ever mentions? Or is it something else?
In short, am I doomed, lazy, or just ignorant of something very important?