You can be calm when the market falls or may sell using margin trading. Why not? I personally think that it is also a good idea. However, you need to analyse first, as any fall may be a part of a brief correction:)
being calm when the market falls is very good, but it's very difficult to get that kind of thing, because basically when you're holding a coin, or other asset, you definitely won't calm down and be in a hurry to sell it.
It's easy said but hard to do. The best thing is to keep your exposure to the market low. This also slows down your wealth building but in times like these you will be grateful. And it is easier to ignore the news or stop refreshing the stats on coinmarketcap.