Most people always had regrets of not buying on dip, when at 3k, coz when BTC was on lower price, they keep looking for lower and expect much lower til price increase and miss the chance. Mind you its a cycle, that keep happening but still missing chances.
This maybe is one of my regrets aside from learning it a bit late (around September 2017 and at that time, Bitcoin is rising very fast). No one knows what is the dip anyways. If the price went to 4k some will say "It will go lower at around 3k for sure and I will accumulate at that price". When it went up they will feel regret and will say "I feel regret that I didn't bought at 4k". Their bias didn't went in their way. Sometimes instinct can help you whether you will buy at that price or not. I have seen Bitcoin plummeted weeks ago and I'd really want to accumulate more but my problem is that I don't have any cash to buy it so I just watched it going up. No hard feelings though
.So far, my regrets was my tokens on miss of selling them on ATH, just hodling till nothing lol, well that's was my newbie days.
I got some tokens too in different bounty campaigns that I joined when I'm still new here and luckily I sold them all but at a lower price but no regrets till this time since their price is lower than the price where I sold all of them. Maybe my biggest regret regarding tokens is that I bought some of the tokens (invested in their ICO) hoping that it will go up in the long run but it went the other way.