Just Calm Down
Nobody wants to hear this. If you’re really upset or emotional about something, someone telling you to calm down is definitely not going to do it. In fact, it’s just more likely to make the person more upset. So no matter how well-intentioned it is, if you’re thinking about saying that, maybe try something else.
It can be hard to know what to say to someone who’s upset, especially if you don’t really understand the reasons why. But no matter the situation or how strange it seems to you, it’s always the best idea to try and understand the other person’s perspective. Or just don’t tell them to calm down.