Many people make mistakes like this despite being said to be old traders, but they don't learn from the many mistakes that occur.
The main problem is not his knowledge but his mentality which really cannot be controlled.
Too FOMO when the price goes up and don't buy again when the price really crashes, even sell at a low price and get a lot of losses.
5 years is not a short time, it is a time that makes a person well shaped by the market and should have good self-control as well as more expert trading knowledge
You are right about the mentality and some are too stubborn to accept that they are wrong and they need to learn more. They think if they have made some trades in profits they are legends but they don't know anything and not accepting their errors can cost them more.
They also fall for every Fomo and they don't care until their wallets liquidated and they got no money, they sell early and don't book profits, for any trader the best practice is to hold coin until the market is good but if we are doing future trading our SL should be placed to save us some money and newbie will be doing opposite, they will be adding more to increase their liquidity range. Still some people don't learn in 5 years and there are traders who became expert only in a year, it depends on the practice, not on the time they have spent.