This is an interesting subject. Several avenues of inquiry are available to us when considering the search for individual fulfillment in American letters of the 19th and 20th centuries. We might ask what authors most clearly present this pursuit in their work. We might ask what individual novels, plays and poems express this theme. We might also ask how central this theme appears to be in American writing from this period when compared to other themes explored over these two centuries.
Some writers that clearly present the pursuit of individual fulfillment are Walt Whitman (Leaves of Grass), Arthur Miller (Death of a Salesman; The Crucible), Ernest Hemingway (For Whom the Bell Tolls; Sun Also Rises; A Farewell to Arms; Garden of Eden), F. Scott Fitzgerald (The Great Gatsby; The Last Tycoon; Tender is the Night), Robert Penn Warren (All the King's Men; Flood), Langston Hughes, James Baldwin (Another Country; Go Tell It on the Mountain; Giovanni's Room; Tell Me How Long the Train's Been Gone), Toni Morrison (Beloved; Sula), Zora Neal Hurston (Their Eyes Were Watching God), Ralph Waldo Emerson, and Ralph Ellison (Invisible Man).
We might break this group down according to literary movements. The Transcendentalist period was certainly marked by the idea that every person was characterized by a personal spirit (or dharma, as this movement was highly influenced by recently translated Hindu texts).
Transcendentalism revolves around the self, specifically the betterment of the self.
This personal spirit could be developed and was the cornerstone of ideas of individuality and individual authenticity. The search for individual fulfillment, for the Transcendentalists, was arguably a defining characteristic of the movement.
Also, many of the writers from the list I have made here are from the Harlem Renaissance. The notion of self-fulfillment here was a counterpoint to a historical dimunition of personhood and potential and so was politically and thematically central to this arts movement.
Some of the other writers on the list here are modernist writers. For some modernists determining the true nature of man was, in a way, an answer that could only be given when the person was tested against extreme circumstances. This mode of individual fulfillment is quite different from each of the first two types mentioned.
We have three distinct ways of interpreting and writing about the search for individual fulfillment here: the spiritually-driven philosophical search of Transcendentalism; the reactive and political search for positive definition and the fulfillment of a fuller picture of an oppressed people in the Harlem Renassiance; the crisis moment of self-definition presented by the modernists.
As a graduate student you know which writers belong to which movement and you also realize that some of the writers on this list were working after these movements had come and gone. The search for individual fulfillment constitutes a definite thread in American letters, connecting writers working today to those from the 19th and 20th centuries.
For a fruitfull inquiry on the subject, it will be best to either articulate a question about how this theme differs or remains the same over time, how it exists within a single time frame, or further articulate and refine the theme to inform a more spefic discussion.