I found this in David McCullough’s 1992 biography Truman several years ago and put it into a little prayer book I made for myself.

Of Truman’s high school years McCullough writes, “He memorized a prayer, one he would say through much of his life,” then quotes this prayer.

Oh Almighty and Everlasting God.
Creator of Heaven, Earth, and the Universe:
Help me to be, to think, to act what is right,
    because it is right;
Make me truthful, honest and honorable in all things;
Make me intellectually honest for the sake of right and honor
    and without thought of reward to me.
Give me the ability to be charitable, forgiving and patient with my fellowmen –
    help me to understand their motives and their shortcomings –
    even as Thou understandest mine!
Amen, Amen, Amen.