I found this:

"...But Obama never literally said this, nor did he write it as a yearbook quote, as the post implies. Instead, the quote is a misleading mash-up of a line in a paper titled “Race and Rights Rhetoric,” which Obama co-wrote with his law school classmate Robert Fisher in April 1991."

“Rising Star: The Making of Barack Obama,” a 2017 biography of Obama by David Garrow, discussed the paper and included the relevant excerpt."

“(Americans have) a continuing normative commitment to the ideals of individual freedom and mobility, values that extend far beyond the issue of race in the American mind,” the excerpt reads. “The depth of this commitment may be summarily dismissed as the unfounded optimism of the average American — I may not be Donald Trump now, but just you wait; if I don’t make it, my children will.”

Source: https://apnews.com/article/fact-checking-181030204951