When something external upsets us we lose connection with ourselves. In fact, each time we get upset, it is because we lose connection with ourselves for some reason or the other. It could be blame (ourself), self-hatred, guilt or whatever, but we lose our connection with our self.
The other person or situation may not even be aware that you are likely to perceive things in your own way and they behave normally. It is us, who find a reason to lose connection with ourselves because when we lose connection with ourselves, we punish ourselves. The internal world is shaken even if the external world is normal. In normal cases, people get shaken when the external is shaken. But we could very well get shaken even if our external is calm.
We just hit ourselves by losing our connection.
Hold on to it. This connection of love for yourself. In whatever circumstance. Then, whatever happens on the outside, the internal is calm. You are loved by yourself. You are connected to yourself.