A search for the earliest use of the term "willpower" in litterature using google books.
The earliest record of the word “willpower “ in literature I’ve found in my research using google books is from a book called "The Approaching Crisis Being a review of Dr. Buschnells recent lectures on Supernaturalism" from 1852.
Of where he writes on page. 146
There are also books around the same era for an example: "Book of Human nature illustrating the philosophy (new theory) of instinct, nutrition, life with their correlative and abnormal phenomena, physiological, mental, spiritual." by Laroy Sunderland from 1853.
On the topic about self-control page 93.
Another book around the same time in the century on willpower. "The problem of American destiny solved by science and history" by the author Charles T. Evans from 1863.
Page 35.
Additional sources:
etymonline.com currently lists the term willpower origin to the year 1874.
Dictionary.com currently lists the term willpower of which origin stems to the years 1870-1875.
Additional notes:
Those earlier ngram records that shows bumps to 1718-1724 and the beginning of the 1800's are bugs in the search engine books or they either have the term in the wrong context and meaning.
Example of a false “bump”:
