Need to refresh your math skills? From basic algebra to calculus and beyond.
I found that the best way for me to internalize a particular subject in hard science is to follow its history by reading the original papers. It's a hard way to do it, but it helps you understand the motivation, the progression of ideas, and the rigor that went into it. Anything else doesn't quite work for me - I feel like I "sort of" understand, but five minutes after I close the textbook (or the YouTube video) the understanding goes away with it.I watched some of these lectures - they're excellent, but a bit too hand-wavy. Without getting through the rigor there's no hope to gain a complete understanding of the subject matter that stays with you forever.