A quote I often think when dealing with a stressful moment, is this one:
I undertook to conquer myself rather than fortune, and to alter my desires rather than change the order of the world, and to accustom myself to believe that nothing is entirely in our power except our own thoughts. - René Descartes
It puts things into perspective.
It then reminds me of these other two:
Everything we hear is an opinion, not a fact. Everything we see is a perspective, not the truth. - Marcus Aurelius
If you are distressed by anything external, the pain is not due to the thing itself, but to your estimate of it; and this you have the power to revoke at any moment. - Marcus Aurelius
We only control our own thoughts. How we react to externalities is well within that remit. Everything else is pointless.