I have quite a few that mean a lot (although quite a few have been written already):
When you're going through hell, keep going (Churchill)
Not my circus, not my monkeys
This too shall pass
When someone shows you who they are, believe them (Maya Angelou)
Success is not final, failure is not fatal: it is the courage to continue that counts (Winston Churchill)
Ships in the harbour are safe, but that's not what ships are for
If you judge a fish by its ability to climb a tree, it will spend its whole life thinking it is stupid
The Invictus poem, especially the last 2 lines - I am the master of my fate: I am the captain of my soul
Comparison is the thief of joy