It's a bit like learning a language, you'd need some almost immediate application to make it past the first steps of a book or course (and/or make that gained knowledge stick). And while it's easy enough to travel to some country where they speak a language you just learned, that's often harder when it comes to mathematics and IT.

3D maths and game development worked out somewhat in the past (I forgot most of it though, and as that was the DOS days it wasn't really more than high school level geometry). But I doubt that I could find enough real noticeable application for e.g. category theory.

(Just being the basis for something often isn't enough. You don't need to know much about physics to hop on a trampoline.)

It helped to have a purpose or something to accompany it, e.g making a game as that would require a bit more math than CRUD apps. This helped me brush up on some (embarrassingly simple) vector math http://natureofcode.com

Indeed. I really just need to relearn most of the mathematics I took in college. Of course, without a job (or project) that demands things like calculus, I'd just forget it all again anyway.

My current job has me messing around with linear algebra and statistical analysis on occasion, though I've found the cobwebs in that part of my brain are rather thick :)

Same - I think it's because OK: I rote memorized how these equations work, but what now? What do I use them in? If I used them in my day to day, I would probably enjoy it more and probably stick to it.

same here man, same here...