For me, the biggest stupidity I have ever faced is losing faith in bitcoin during difficult times. I knew about this market quite early for many people at that time when bitcoin was mostly seen as the scam of the century and most of it appeared with shady practices and was not as popular and accepted as it is now in.
I sold some of my bitcoin just to pay for partying, and now I find it worth it because I realize it's not too late to choose to trust bitcoins.
Remember one thing: you sell bitcoin when you think you have achieved some goal, but then you realize that if you don't sell it, it won't affect your life and you just keep it won.