"Harden up" is one of the most callous things you can say to someone who's having a bad time. At the very least, they need someone to talk to, not someone who dismisses their concerns and suffering. Often, it's enough to prevent them from taking their own lives.

Those who suggest to others to harden up should start by example. Parasites should take the chance at symbiosis !

I think it high time the term 'mental illness' is being looked a little closer. Not getting what you want for nothing is being selfish & not an 'illness'. Being told time & time again to pull up your socks & your finger out is not mental illness.
Being ignored by authorities or being let down by incompetent bureaucrats causes stress leading to mental illness. Abusing drugs, abusing your body, abusing your privileges etc leads to physical illness not mental.
Not accepting your own limitations is not mental illness but thinking it is your Right to demand from others what you can't produce yourself will eventually make others getting sick of you !