I've always felt inspired by quote by the great Alexander Pope-
"Act well your part, there all honour lies".
I feel this is an excellent mantra to live by.
Also, the following, by Sarah Williams has helped me in the past in times of darkness and I feel it gives one a sense of cathartic mortality-
"Though my soul may set in darkness, It will rise in perfect light. I have loved the stars too fondly, To be fearful of the night."