There are several synonyms for the phrase "disappointment of proponent". These may include dissatisfaction, frustration, vexation, disillusionment, and letdown. These words usually convey a sense of disappointment that arises when someone's beliefs or expectations do not align with reality. Additionally, one can use words like deflation or discouragement to describe the sense of diminished enthusiasm and energy that often accompany disappointment. It's important to acknowledge disappointment when it occurs, as it can be a valuable learning tool, but it's also crucial to find ways to move on and channel that disappointment into productive action.