We cannot turn back the hand of time, there is one thing which I have learnt about life, to take our our past experience as something that's preparing us for the greater opportunity ahead, regret is one things most times I don't like doing, so you regretting that you sold off your Bitcoin on the earliest stage before the bull run is actually something anyone can do, but let me ask this question, if everyone who bought, mined and hold alot of Bitcoin hold it all off without selling till now do you think Bitcoin adoption will be outreach the way it has right now. And another thing their is that you were actually not informed or you have a very little believe regarding what the price of Bitcoin might turns out to be in times to come. And at the very moment when you sold your Bitcoin the price and the funds gotten could have actually solved a pressing need.
That being said, regret something is one thing I see as suppressor, like it might actually hold you down from achieving something things which you could have, because you have really moved passed your past which you call a mistake.