So I've been learning maths from online video lectures and books for some time now and, as is probably quite natural, I have forgotten some of the stuff I've already read through. That'd be mostly some galois theory and "Analysis on manifolds" by Munkres.
My question is - is it more effective to read through those books once again, to make sure I remember as much as possible or should I just keep going, reading books about further, more advanced topics, assuming I have the basics to understand them?
If you want to engage with the actual subject, the answer is "all of the above". That is, yes, regular review of things one's already read is necessary, if only because the human mind is fallible, forgetful.
Yet one should not allow understanding one's own forgetfulness or even incomprehension to inhibit moving forward. This is true in a much stronger sense than nearly all novices perceive, namely, that the sense of many "lower-level" things can only be understood genuinely at some later moment, in terms of more sophisticated concepts (which, duh, srsly, ppl, were created for that understanding). That is, thinking that "strict logical development" is a good virtue to practice is ... misguided.
That is, mathematics is not "logic" (although "logic" is a good thing in itself...), thus, a dynamic engagement with it, if you like it at all, is the only real way. Engagement "according to rules" is artificial, probably not so good. Maybe harmless, at best. Tastes differ. "Your mileage may vary."
The last note is that there is the insidious pretense/mythology in mathematics that it is "objective" or somehow "trans-human/worldly". This conceit clouds-the-minds of many students, and is dangerous. People are people. Not transcendental, even if we might like to be. :)