Issac Asimov famously hit a "breakpoint" at integral calculus. I myself remember "sweating blood" to get definite integration; unfortunately never got a chance

Issac Asimov famously hit a "breakpoint" at integral calculus. I myself remember "sweating blood" to get definite integration; unfortunately never got a chance to do multiple dimensions. Or take organic chemistry. I have a particular type of memory inherited from my paternal grandmother, anything I can fit into an organization of related things I retain well. And I have really really good 3D visualization. First term organic chemistry was easy, and I didn't have to use physical models more complicated than my fingers and knuckles. It's well known that at minimum this is a lot harder for a lot of people. Heck, it's an appropriate weed out class for premeds, appropriate in that it tests both of those capabilities which will later be needed to learn medicine.