I think that happens because it is hard to just do a search of everything that I know.

There are tools you can use to solve this problem! Have you tried mindmapping everything you know, e.g. with FreeMind?

whenever I think "what do I know that I can explain to people that isn't explained well elsewhere?" not much comes to mind.

At least in math, many topics are explained well at some high level but not explained well at a lower level. There's always more work to be done explaining math to the general population; the gulf between what could be explained and what has already been explained is absurd.