My biggest regret is when I got introduced to this back in 2012, I tried to buy £200 worth, the platform was down and I never bothered, just recently woke up to it again at over £4k per coin, it is a physiological thing where I don't want to buy at that price.
Same thing with me, but in my case, it was the year 2015 where bitcoin price is around $200 to $500. I bought almost 1btc and lost it in HYIP's and Gambling, I just don't bother about having an ROI, I just tried to use it for fun because it was something new. I did not try to research about it, maybe I did, but I think only for a small idea, and just bought and spent it directly. Then after that year, I stopped it, and come back this year when I found out what it has become right now having the regrets for what I did with my bitcoins during that time.