F. Scott Fitzgerald : "Never confuse a single defeat with a final defeat."
Success is sweet and sweeter if long delayed and gotten through many struggles and defeats." -Amos Bronson Alcott
This can apply to weight loss, but it's not really about motivation, but more of philosophy on life:
"Every time you wake up and ask yourself, "What good things am I going to do today?", remember that when the sun goes down at sunset, it will take a part of your life with it."
Indian proverb
Build A Bridge... And Get Over It... If I've messed up, fallen short of my healthy eating, slacked off on my exercise instead of beating myself up over it.. I build a bridge.. lol
I don't know if it counts as a quote, but when I'm working out I frequently tell myself I can do anything for 1 minute when I'm trying to push myself to go just a little bit harder.