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 .
You sound like a trader, which coin are you even talking about and you also encourage buying at the dip and selling when the price increase, waiting for the price of Bitcoin to drop before you can accumulate is not a good strategy expecially for new investors.
Also with the DCA strategy you will not be waiting for the price of get dip before accumulating since you will be accumulating either every weeks or months irrespective of the price of Bitcoin and HODL for 4-10 and above.