Ugh, I need to learn more math. How do I even start? I know multi variable calculus, I know the basics of linear algebra, and I know Fourier transforms. Yet this article is half gibberish to me.
imo abstract algebra is pretty much the gateway to most modern math. it well feel at first like it's a lot of machinery without purpose, but understanding groups rings and fields well opens the door to topology, advanced number theory and a bunch of the rest of math. the other option would be to learn some real and complex analysis, but imo the algebra side is where a lot more of the cool stuff is.
If you are any sort of programmer "Linear Algebra" is going to be the best bang for your buck. You can have a look a some of the books of Gilbert Strang or his online courses. Beyond that the classic books in "Abstract Algebra" are those by Serge Lang or Jacobson. Either or might be too difficult and maybe not worth it.
I always tell people to study integration correctly. By that I mean measure theory, with the Riemann integral taught in schools you can only go so far.
With those two you are in a good position to tackle functional analysis. The classic book on the subject is that of Walter Rudin but maybe it too hard for self learners. A better alternative might be the open course at MIT:
Functional analysis is terribly important in practical areas like signal processing, quantum mechanics or partial differential equation in general. so it might be worth your pain.
There are many ways you can go from there.
I can't leave without recommending one more things. If you are curious of advanced mathematics as a tourist the three books by Avner Ash and Robert Gross make a great read