How can we calm ourselves down?
'Just breathe' is not that well understood when in dread or anxious. The brain is not working well. We have to instruct people or think more simply, more specifically.

Try to inhale to your belly first, then fill your chest, exhale from your chest first then belly. Make your exhalation longer then your inhalation.
Do it again 5X, stay with 2 stages if your feel ok there.

If you want add 'inhale to your clavicles/upper chest as a 3rd stage' if you wish.
'........  inhale your belly first, then fill your chest, then to your upper chest/clavicles, exhale in reverse order. ...... '
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I don't suffer any of the things in the subject line, maybe that's because I lift weights 3 to 4 times a week, do regular cardio, do sauna and ice baths and almost never go to sleep after 10pm