My biggest mistake so far is to sell my bitcoin's when it was at $15k not knowing that will hit $19k but i have bought back from the $6k so its not that too bad because i gained more bitcoins buying at the bottom,lets hope that bitcoin will going to bull run again so that these altcoins will grow to whenever these bullruns happens it shows positive outcome for the altcoins.
Selling your bitcoins at $15k (presumably for a profit) and then buying back in at $6k doesn't really sound like much of a mistake. Just because you didn't sell at the ATH, don't look at at it as a mistake, look at it as locking in profits, which is absolutely a good thing.