I pretty much have learned to live my life by this quote, because life throws you curve balls, and the best thing you can do is live in the now, with what you have, and not think about what you don't....
I don't have just one favorite quote. Here are a few I really like:
"The most important thing in communication is to hear what isn't being said" Peter F. Drucker
"If you want to walk on water, you've got to get out of the boat" John Ortberg
"We are all called upon to do small things with great love" Mother Teresa
"A friend is one to whom one may pour out all the contents of one's heart, chaff and grain together, knowing that the gentlest of hands will take and sift it, keep what is worth keeping and with a breath of kindness blow the rest away." Arabian Proverb