There are no problems, only
opportunities. - Mother Theresa (Remembering this has truly changed my attitude on multiple occasions. Always seems to be true, though we so often don't realize it at the time.)
Your spirit is your true shield. - Morihei Ueshiba, founder of aikido (I have a little book called "The Art of Peace" and I sometimes start my day by randomly opening to a page. It is amazing how often this is the page that pops up!)
If you want to change the world, start with yourself. - Lots of people have said this, I think. It most certainly helps for me when the big problems of the world seem overwhelming.