This quote is taken from the last U.S. Senate prayer Peter Marshall wrote as chaplain before he died at age 46 in January 1949:

“Deliver us, our Father, from futile hopes and from clinging to lost causes, that we may move into ever-growing calm and ever-widening horizons. Where we cannot convince, let us be willing to persuade, for small deeds done are better than great deeds planned. We know that we cannot do everything. But help us to do something. For Jesus’ sake, Amen.”