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.
for me the time you buy doesn't really matters but when you sell, if you have a plan of selling when the value is higher then no matter what you spent during your purchase you are sure of making some profit because you already have a plan and you are committed to this plan not minding what the prevailing circumstances are, and then when it is convenient for you then you carryout your transactions, perhaps this is where the long term approach comes in.
You should buy when the price drops and you cannot buy when the price increases. This is a wrong method because you may think that now the price has dropped, now is the time to buy, but it turns out that when you buy thinking the price is low, after a while the price has dropped even more, then you will be disappointed. Therefore, no one can accurately say when the price will drop, so do not be too afraid of the price drop or increase, but invest a certain amount of money regularly every week or month, no matter how much the price rises or falls. DCA (Dollar Cost Averaging) solves this problem. In this way, you can buy assets at an average price.
Suppose, you DCA an amount of $100 in Bitcoin every month. If the price drops, you will get more Bitcoin for that $100. If the price increases, you will get less Bitcoin for the same amount. In this way, your average price in the long-term will come close to the average market price. The possibility of profiting in the long term is high. History says that Bitcoin is a strong asset with an upward trend in the long term, so if you only want to buy when the price drops, you may miss the opportunity. Because sometimes the price may not fall as much as before. By doing DCA, you will deposit regularly without worrying about price fluctuations. In short, this mindset of "buy if the price falls, don't buy if it rises" will put you in the trap of market timing. DCA strategy will make you a disciplined investor without depending on the ups and downs of the market.