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.
I think it is right that if you buy at a low price and later sell when the price increases, then the profit will remain. There are many who do not want to buy when the price decreases but if we pay attention, we will see that when the price of Bitcoin decreases, it increases again, in which case you can invest in Bitcoin and in the long run there will be a huge profit opportunity. The right time for you to sell is when you get the price you want. If you get the price twice as much as you expected now, then sell.