"You have power over your mind, not outside events. Realise this and you will find strength" - Marcus Aurelius
These words have proven incredibly valuable throughout my life thus far, and I'll explain why.
Whenever I think about the world and all its problems, I am overwhelmed. I find that reminding myself of Marcus' words helps to shrink the size of the world in to a tiny, manageable sphere. I can see where my efforts are best delegated, rather than having my head spin. Sure, there will be events beyond my control that will affect me negatively, but there is little use in being worried about them -- something I have to constantly remind myself of.
Rather than banal platitudes of 'just be positive', or something equally as useless in its nebulous abstraction, Marcus' words have me focusing on how my thoughts can help me do the best with what I have.
It's okay to be powerless, in the grand scheme of things, if your input cannot alter the result. Realising that you determine whether you to live in a fuliginous cesspool, or a non-stop golden opportunity dream world, helps so much in preparing to face the world.
So when reality grows tough, I always first think 'what can I do?', rather than wallow in the despairs of misfortune. It's about having a realistic, positive interpretation, rather than merely being ignorantly positive.