“I am only one, but I am one. I can't do everything, but I can do something. The something I ought to do, I can do. And by the grace of God, I will”

Statement published in A Year of Beautiful Thoughts‎ (1902) by Jeanie Ashley Bates Greenough, p. 172, Third statement for June 11. This has often been misattributed to Helen Keller in some published works since at least 1980, perhaps because she somewhere quoted it.
Variant:
I am only one,
But still I am one.
I cannot do everything,
But still I can do something;
And because I cannot do everything,
I will not refuse to do the something that I can do.
The Book of Good Cheer : A Little Bundle of Cheery Thoughts‎ (1909) by Edwin Osgood Grover, p. 28; also in Masterpieces of Religious Verse (1948) by James Dalton Morrison, p. 416, where it is titled "Lend a Hand"
Variant: I am only one, but still I am one. I cannot do everything, but still I can do something; and because I cannot do everything, I will not refuse to do something that I can do.

The Wild Boys (1971)

Variant: My faith demands - this is not optional - my faith demands that I do whatever I can, wherever I can, whenever I can, for as long as I can with whatever I have to try to make a difference.

Source: Ghost Town

Source: The Happiness Project: Or Why I Spent a Year Trying to Sing in the Morning, Clean My Closets, Fight Right, Read Aristotle, and Generally Have More Fun

§5, p. 84
Knots (1970)

regarding acknowledging Elizabeth Warren as Native American 2018-10-15 Trump promised $1 million to charity if Warren proved her Native American DNA. Now he’s waffling. Amy B Wang and Deanna Paul Washington Post https://www.washingtonpost.com/politics/2018/10/15/trump-dared-elizabeth-warren-take-dna-test-prove-her-native-american-ancestry-now-what/
2010s, 2018, October

Source: On challenging stereotypes in “Researcher Nadia Barhoum interviews Rabih Alameddine” https://belonging.berkeley.edu/alameddine (Othering and Belonging Institute at UC Berkeley)

The Black Coat (2013)