[Nat]

Not sure if these count as creeds and they are too short to be mottos, but I have embraced them for a long time:

1. O me! Oh Life. O the questions of these recurring. O the endless trains of the faithless. O cities filled with the foolish! What good, amid these, O me, O life? The answer: That life exists and identity. The powerful play goes on and you may contribute a verse.

- Dead Poet's version of Walt Whitman quote

and

2. I went to the woods because I wanted to live deliberately. I wanted to live deep, and suck out all the marrow of life. To put to wrap all that was not life, and not when I had come to die, discover that I had not lived.

- Dead poet's version of Thoreau quote

I'm also partial to the Princess Bride quote:

Life is pain, highness. Anyone who says differently is selling something.