Am I in charge of my own mood?

Someone once told me that I should never let other people bring me down. I'm in charge of my own happiness.

But how can I not let other people bring me down when they work so hard at making sure I am aware of the fact that I just ruined their day completely because of something I have no control over? Because of my circumstances another person is angry with me, and making sure I know it, because it doesn't work for them.

Yea. Someone else has brought me down. I'm in a rotten mood.