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.