Check your jaw. Most people find it clenched, and most have no idea how long it has been that way.
Bracing is cheap in the moment and expensive over a day. The body takes up a position during something difficult and then simply forgets to put it down — the way a hand can stay curled around a pen long after the writing stopped.
Allowing is the honest verb here. You can't order a muscle to release; you can only stop asking it to hold. Unclench the jaw, let one breath out slower than you took it in, and the shoulders usually follow without being asked.