One of my favorite sayings has been hanging over my desk for years. It is a Helen Keller quote.
I am only one, but I am still one.
I cannot do everything, but I can still do something.
And because I cannot do everything I will not refuse to do the something that I can do.
I try to live by this concept. I am one person and if I can cause a positive ripple through the world around me then my existence here is a good one.