To be honest, the text that touched me most and helped me in my darkest moments is a poem, "Invictus" by William Ernest Henley. It's not really (exclusively) about mental health, but rather about never giving up, fighting against the odds and keeping your head up in even the direst circumstances.

Invictus

Out of the night that covers me,

Black as the pit from pole to pole,

I thank whatever gods may be

For my unconquerable soul.


In the fell clutch of circumstance

I have not winced nor cried aloud.

Under the bludgeonings of chance

My head is bloody, but unbowed.


Beyond this place of wrath and tears

Looms but the Horror of the shade,

And yet the menace of the years

Finds and shall find me unafraid.


It matters not how strait the gate,

How charged with punishments the scroll,

I am the master of my fate,

I am the captain of my soul.


This poem always sends chills down my spine. It's not just aesthetically beautiful, it's a mindset. It's the only poem I ever memorised by myself, without being told to. And I didn't even need to repeat it over and over, it just burnt itself into my skull as soon as I read it for the first time.