You can do this in private, or anywhere where you can take a moment to breathe in a time of need.
Close your eyes and imagine the negative feelings as a a black ball. See it before you. With each breath out, watch the ball turn white. With each breath out, feel your self get calmer. Continue this until the ball is completely white.
Once it is totally white, watch this ball leave your sight. Feel your negative energy go with it, and breath in the new calming sensation.