The best time for buying is at low price and I think if you sell your coins with profit then you should not regret after Fluctuations in coin price because you have already taken your profit so ready to make another step instead of regret about past.
If you are in doubt about other coin then just buy bitcoin now and don't sell until you achieved your target. This time is not suitable for selling but if someone is in huge profit and he cannot hold longer then selling should be preferred. Understand the market and accept your profit but I think if someone is selling their bitcoin without waiting more so he will miss the opportunity of having huge profit in Bull season.
regrets after selling Bitcoin mostly happens because no one can perfectly predict the coin market.
Prices move up and down it's just based on supply demand and sentiment.if after buying,the price goes down immediately just wait untill it's goes up if you are not urgently in need of money at the time,you just can't predict this things.but it's always save to buy coins when the prices drop .
Your statement here is wrong, Being urgently in need of money should not be the determinant for selling bitcoin, people regret selling bitcoin because they sold early and because they chose to trade with bitcoin rather than invest in it which is a financial mistake, no one should be trading with bitcoin on any normal circumstance instead just keep accumulating with DCA, waiting for the DIP when you can DCA is also a financial mistake as it shows a tendency to sell immediately the price goes back up so don't be telling people it's okay to sell if they are urgently in need of money, this is why having an emergency fund in important as an investor so you don't ever have to sell too early.