Take three slow, deep breaths. Continue with your day. Repeat as needed.
While you'll often hear the advice to take three deep breaths when you're upset, it is difficult to convince yourself to do this while your mind is spinning and your emotions feel like broken glass.
So, instead, get in the habit of taking three breaths before brushing your teeth or while waiting for the kettle to boil or some other low-emotion experience. When you have this habit, it will feel far more natural to do deep breathing when you are tense.