The main lesson is to understand the market, to know in which direction it goes, in what phase we are, why are we there, technical analysis? Fundamental? both fail, the main thing is to understand the market and only understand studying books, such as Wyckoff, Livermore, who were first-rate speculators, also Buffet who is a sensational investor.
I wouldn't put too much emphasize on all theoretical. I learn everything from this community and searching around the web. Technical analysis though is difficult at the beginning and you really need to learn it my practicing.
Also, if you become a successful trader and started to make big money then security is the next big thing to learn. Invest on a hardware wallet and don't keep big amounts in trading exchange so that at least you won't get mad if those exchanges get breached or something.