Math alone is GIGANTIFIC! To working mathematicians pre-university math is not fundamental. I hate saying this, but high school math is not even considered "actual" math. Even undergrad curriculum is just a sampling of pieces and bits of what's to come in grad school where you get to look at things a bit deeper. But even after grad school it's still very hard to say you know all the fundamentals of math. You might come out a capable differential geometer (after grad school), but that doesn't mean you'd know the, uh, the "fundamentals" of, say, number theory past the undergrad level (which, frankly, is not enough to say you know the fundamentals of the subject).

That said, not all is lost. All aspiring mathematicians must start somewhere. You can start here [0]

[0] BOOK OF PROOF by Richard Hammach.

https://www.people.vcu.edu/~rhammack/BookOfProof/

Prerequisite for reading [0] is math at the level of, say, 7th-grade. Basically, all you need to know is some elementary algebra. The material in [0] is what all math people must know.