The phrase ‘whatever doesn’t kill you makes you stronger’ is false.
I love the sentiment and intention behind it.
Most of the time I hear it used, it is true.
I typically hear it used relation to a specific stressful event.
For example: a hard workout, a difficult conversation, or an injury.
However, I fear there are some out there who apply this phrase to chronic stressors.
For example: daily anxiety, poor diet, or sleeping too little.
These types of chronic stressors are not making you stronger.
So perhaps it would be better to say:
‘Whatever doesn’t kill you today will make you stronger tomorrow if you recover from it properly.’