I’ve moved most of what was on this page to https://learning.subwiki.org/wiki/Difficulty_of_learning_mathematics.
https://www.facebook.com/qiaochu/posts/10155226053470811?comment_id=10155226054835811&reply_comment_id=10155226905650811&comment_tracking=%7B%22tn%22%3A%22R7%22%7D
Terry Tao and Tim Gowers also talk about similar things.
https://jonahsinick.com/the-truth-about-mathematical-ability/