Some things I do to calm down are:
Put an ice pack on the back of your head. This will cool down your brain and reduce activity in your limbic system, where emotions happen.
Do a task with simple, clear steps. I like baking. I follow the recipe, and it’s easy enough that I don’t get frustrated, but it requires enough concentration that I don’t have room to think about other things. Cleaning also helps.
Engage your five senses. This will remind you what is real and what isn’t. Listen to soothing music. Smell something pleasant, like an essential oil. Suck on a hard candy. Look at pictures of things you enjoy. Feel something with an interesting texture, like a piece of Velcro or a stress ball.
Try to levitate something with your mind. You obviously won’t succeed, but it gets you to focus all of your attention on a single point outside your body, instead of on your thoughts. I like to use this on airplanes or in public places, because it doesn’t require you to move at all. It looks like you’re just daydreaming to observers.
Listen to a guided meditation recording