I believe it's a really personal thing, but I'll still give you my 2 cents.
I think that trying to calm down when under big stress is often not gonna work out if the situation is really heavy. Instead, accept your current mood and channel all those energies onto something else. You know, hit a punchball until you can't feel your knuckles, go running for 2 hours, listen to "angry" music and sing along. Find a way to pour out.
If you are unable to do that, try masturbating. It really does wonders for me at times, it completely erases both good and bad thoughts off my mind for a while. Just don't rush it, take time for yourself!