Resentment is not a core emotion, it is a complex emotion that often indicates an unmet need. 

Plutchik, R. The Emotions. New York: Random House.

Brene Brown 2021. New York: Random House. Atlas of the heart : mapping meaningful connection and the language of human experience. 

Plutchik proposed a psychoevolutionary classification approach for general emotional responses. He considered there to be eight primary emotions—anger, fear, sadness, disgust, surprise, anticipation, trust, and joy. Plutchik argues for the primacy of these emotions by showing each to be the trigger of behaviour with high survival value, such as the way fear inspires the fight-or-flight response.

Plutchik, R. (2000). Emotions in the Practice of Psychotherapy: Clinical Implications of Affect Theories. Published by APA.