Ask HN: What should I do to learn computer science fundamentals?

I feel like I don't really understand some of the fundamental CS stuff, and I'm just cargo-culting. I mean, I can write some stuff and I have a decent understanding of recursion, data structures, and some low level stuff about processors, but I feel a general sense of "what am I doing" while writing any code and I don't really know what I'm missing. I don't know, maybe this is just impostor syndrome or something, but I definitely am missing some fundamentals. Does anyone have recommendations about what to learn and how? Thanks!