I've been wishing for years that someone would maintain a "dependency graph" for mathematical concepts. I think Khan Academy tried to do something like this at one point but took it down a long time ago. As long as we're far enough from the bleeding-edge of research topics, I feel like maths is the one field where this might be possible to do really well. You should be able to point at something you don't understand and trace backwards in a very granular fashion until you hit a concept you already know/understand, and then learn what you need to fill in the gap!