I long to accomplish a great and noble task; but it is my chief duty and joy to accomplish humble tasks as though they were great and noble. It is my service to think how I can best fulfil the demands that each day makes upon me, and to rejoice that others can do what I cannot. Green, the historian, tells us that the world is moved along, not only by the mighty shoves of its heroes, but also by the aggregate of the tiny pushes of each honest worker, and that thought alone suffices to guide me in this dark world and wide. I love the good that others do; for their activity is an assurance that whether I can help or not, the true and the good will stand sure.

Helen Keller (1880-1968) American author and lecturer

“Optimism,” Part 1 (1903)
    (Source)

Often paraphrased as: "I long to accomplish a great and noble task, but it is my chief duty to accomplish humble tasks as though they were great and noble. The world is moved along, not only by the mighty shoves of its heroes, but also by the aggregate of the tiny pushes of each honest worker."