Which word describes a keen feeling of mental unease from disappointment?

The term that accurately describes a keen feeling of mental unease stemming from disappointment is "chagrin." This word specifically refers to a feeling of vexation or embarrassment at having failed or been humiliated. It captures the complex emotional response one experiences when expectations are not met, resulting in disappointment and a sense of discomfort or distress.

In contrast, “ecstasy” conveys intense joy or delight, which does not align with feelings of unease or disappointment. “Agony” denotes extreme physical or mental suffering but lacks the nuance of disappointment that chagrin encapsulates. “Apprehension” implies a sense of dread or anxiety about what might happen in the future, rather than a reaction to past disappointments. Therefore, chagrin is the most fitting term for the specific feeling described in the question.