/sci/ - Science & Math

So lately I've been interested in learning more complex math and I've taught myself trigonometry and calculus (high school senior here). I've learned quite a bit of calculus and I want to know if you guys can point me to another subject I can learn? I thought of physics but I'd rather have a teacher for that. Any answers would be great, thanks

What is best time period to read difficult/technical books, like is it better to spend 4 hours straight reading it or do 30 minutes then take a break?

I'm trying to read a few mathematics books but find myself struggling to stay focused on it, the content is interesting but the sentences are all complex and I always end up having to read them over and over again to even understand their meaning.
I'll give an example:
"Among all closed plane curves of the same length the circle encloses the largest area, and among all closed surfaces of the same area the sphere encloses the largest volume."

Now this one is fairly straightforward, but I still had to read it about 3 times to know what it was trying to say.
By the time I get to "of the same area" I forget that it was talking about closed plane curves, and same with the next bit and closed surfaces.
It can be quite frustrating

It might seem weird to ask here, but where do you guys go to discuss mathematics?

There are some intelligent people on /sci/.. but there's not really much going on usually. Majority of threads are either troll threads, humorous anecdotes or people asking for help. The other threads are just things that aren't to my interests, and it's rare that I actually find a decent mathematics thread.

You know of any better places?
Not necessarily an imageboard, in fact the fast paced format is probably half the reason why /sci/ isn't as good as it could be.

I want to become a mathematical autodidact.
How?

I am used to following curricula, and no idea in which order things should be learnt.

Going to study it at university anyway so I think it would be pointless to just learn what I would learn there.

What would be beneficial that isn't taught at university? (or perhaps will expand upon what is/give me an edge)

More interested in pure than applied.