Focus as much as possible on the underlying concepts from any math topic and how they connect to other concepts. Try to boil these concepts down to the most simple, clear form that makes sense to you. Test your boiled down conceptual understanding by applying it to related exercises/problems you haven't tried before and seeing what happens.

For each sub-topic, most math books give you the tools first and then teach you problems they should be used on. Read the problems first, and think how you might solve them (don't expect to figure it out, but if you do, great!). Then, go back and learn the tools, trying mostly discern the "how" and the "why" as opposed to the "what". Math is all about "how" and the "why". As some motivation, whenever a new thing clicks, it is very satisfying! :) But it definitely is a tough process.

Good luck!

'discern the "how" and the "why" as opposed to the "what". Math is all about "how" and the "why".'

Could you pls expand on the diff between the "how" and the "what"?

Textbooks and exercises. Avoid shortcuts such as online tutorials or anything with "for hackers" in the title.

I've been using Khan Academy to refresh my skills and it's been really helpful, but do you have any recommendations for textbooks?

I'm 10 years out of school and haven't really needed to flex my math muscles in years.

If you're past calculus, I'd say start with Sheldon Axler's Linear Algebra Done Right (it has the most immediate applications, and linear algebra is essential for almost all higher mathematics).

Any particular "math" you're interested in?

If you just want to get your foot in with pure math I'd recommend studying basic abstract algebra and analysis at the same time. For abstract algebra look into Hungerford (Intro, not his grad text), for analysis, maybe Rudin, or Kolmogorov and Smirnov.