“i am always doing things i can't do. that's how i get to do them.”

Attributed in Civilization's Quotations : Life's Ideal (2002) by Richard Alan Krieger, p. 132, and many places on the internet, this was actually stated by Vincent van Gogh in a letter to Anthon van Rappard http://en.wikipedia.org/wiki/Anthon_van_Rappard (18 August 1885) http://vangoghletters.org/vg/letters/let528/letter.html, also rendered "I keep on making what I can’t do yet in order to learn to be able to do it."
Misattributed
Variant: I am always doing that which I cannot do, in order that I may learn how to do it.

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"

Controversy
Song lyrics, Controversy (1981)

At the Mahon Tribunal on 20 September 2007. Planning Tribunal Transcript http://www.planningtribunal.ie/images/SITECONTENT_738.pdf planningtribunal.ie. 2007-09-20.

Bolton Partners v. Lambert (1889), L. R. 41 C. D. 300.

§5, p. 84
Knots (1970)

"How Am I Different" (co-written with Jon Brion) · Video https://www.youtube.com/watch?v=iW7gUnN7yxo
Song lyrics, Bachelor No. 2 or, the Last Remains of the Dodo (2000)