We are only here to invest, buy low sell high. so the common mistake people make is definitely buying at a low price but usually selling at a not-so-high price, and then that coin goes up, and we regret it again. I think everyone has experienced this feeling. The market is unpredictable so that's not surprising, and I believe those mistakes will be repeated in the future.
Yes it's true I also felt regret as you mentioned where at that time I bought 2 ETH when the price was still cheap and planned to hold it for the long term but a few months after buying suddenly the price of the coin increased and my portfolio increased by about 20x because I was so eager I finally decided to immediately release everything, but it was a pity after I sold it but the price of the coin went up again higher.
I prefer to be in your shoes rather than not losing back all market gains to market itself, at least you somehow managed to take profit and liquidate position. Buying something with crypto gains makes it harder to reinvest into the same cryptocurrencies and it is bad feeling when you give it all back + main portion of capital to market during bear market cycle.