The quote is from "A Girl I Knew," a short story by J.D. The story is pretty typical Salinger -- ambiguous and sad and philosophical -- and it's one of my favorites. Salinger had a beautiful way with simple words.
This is the most important thing to remember of all! Why, oh why is it so hard for me to have patience! so that's something I really need to work on in my personal life. Having patience and keeping on going even when I mess up a little bit.