These weren't my mistakes, but it happened on other people:
1. Hold your coins in centralized exchanges, if the CEX got hacked, they lost all of their coins.
I think for those who are trading, this can't be help since they have to put their funds on the exchanges in order to put a buy wall or a sell wall. But we should keep our fund out of the exchange if we are not trading and never use exchanges as banks.
With regard to exchange, I did make a mistakes by not activating security features such as 2fa that lead to my funds being hacked.
2. Buy shitcoins, instead of making money, they're in loss.
Not entirely true, there are instances that shitcoins gives a person a better profit than Bitcoin. What shouldn't be done is holding shitcoins because it often ends up worthless. My experience on this is when holding shitcoins one must be always updated and monitored since one missed opportunity will lead us to hold worthless tokens. I have a stash of 3k shit token that were priced at most $20 each during the token's hype but failed to sell them because I failed to monitor the market and did not sell when the price was profitable and later the shit token become worthless because it was abandoned by the developer wich makes me end up holding worthless token.