The concept of "individuation" originates in Carl Jung's theory of psychology and is defined by Collins Dictionary as "the process by which the wholeness of the individual is established through the integration of consciousness and the collective unconscious." What this means, in brief, is that while our psyches represent a whole unique individual, we must internalize the meanings of our imaginations (conscious) and our sleep-dreams (unconscious) in order to integrate the various parts of our psyche by harmonizing our conscious and unconscious expressions of self.
Through listening to the messages of our dreams and waking imagination, we can contact and reintegrate our different parts. The goal of life is individuation, the process of coming to know, giving expression to, and harmonizing the various components of the psyche. ... through a union of conscious and unconscious ... (D. Daniels, Sonoma College)
Individuation is the process of making one's psyche whole, unified, harmonized and integrated by knowing and expressing the true nature of ones intellect, emotions, abilities, sleep-dreams and desires of imagination: the conscious and the unconscious. Since Alec is murdered by Tess; since Tess is executed for murder because she goes mad at Angel's return; since Angel all but dies in Brazil in a mistaken pursuit of his thwarted dreams, it is difficult to suggest that there is evidence of strong individuation in Tess of the d'Urbervilles.
Yet, if we take short segments of Angel Clare's life and Tess's life (we'll ignore the short segment of Alec's life as an evangelist since he denied it as a false impulse), we might find examples of attempts at individuation. One of the keys to individuation is that it succeeds: it harmonizes, integrates, unifies and makes whole the psyche and life.
When Clare desired to study at Cambridge but not to take Holy Orders as a clergyman, he might be said to be on the track of individuation by combining what he knew of his conscious desires with what he knew of his sense of personal integrity:
[Clare to Vicar Clare] "I should like to say, once for all, that I should prefer not to take Orders. I fear I could not conscientiously do so."
When Clare recouped from loss resulting from his father's refusal to send him to college at all if not to take Holy Orders, his pursuit of music and an apprenticeship education in the important aspects of farming to provide himself a respectable and creditable profession, he might be said to be on the track of individuation. His individuation fails when he cannot separate the religious teachings he has already rejected from the shocking revelation of Tess's seduction and childbirth.
When Tess set her mind to building a respectable life for herself in a new land as a dairymaid following her child's death, she might be said to be individuating. She develops this further by attending to Clare's manners of speech and his remarks about philosophy and theology. While her desires were brought to naught by the death of Prince and her introduction at the Stokes-d'Urberville manor as a poultry keeper, she adopted new desires and sought to pursue a harmony between the new and the old. The process of individuation ended when she succumbed to Clare and accepted his pleas that she marry him even though she knew both consciously and unconsciously that she'd never keep her guilty secret and their union could never succeed.
[Tess to Clare] "Don't ask me. I told you why—partly. I am not good enough—not worthy enough. ... Your friends will scorn me."