It's an old story and very probably you've heard it before. But, for some reason, I thought about it again the other day:
An old man was walking along the beach, when he came upon a part of the sand where thousands of starfish had washed ashore. A little further down the beach he saw a young woman, who was picking up the starfish one at a time and tossing them back into the ocean. "Oh you silly girl," he exclaimed. "You can't possibly save all of these starfish. There's too many." The woman smiled and said, "I know. But I can save this one, " and she tossed another into the ocean, "and this one", toss, "and this one..."
There are a number of versions of this. In one version, the creatures being saved are seahorses and the person tossing them back is a little boy. The old man says, "What you're doing doesn't matter," and the little boy says, "It matters to this one."
I like to remember this story when I'm feeling discouraged for any reason.