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.
We should practice to ourself how to stay calm whenever the market is fluctuating because this is what makes you get failed in future. Here in Cryptocurrency we should learn how to play our emotion without being so attached in the market, be relax and think wisely everytime the market falls.