From Wikipedia;

Disappointment is the feeling of dissatisfaction that follows the failure of expectations or hopes to manifest. Similar to regret, it differs in that a person who feels regret focuses primarily on the personal choices that contributed to a poor outcome, while a person feeling disappointment focuses on the outcome itself. It is a source of psychological stress.

The study of disappointment—its causes, impact, and the degree to which individual decisions are motivated by a desire to avoid it—is a focus in the field of decision analysis, as disappointment is, along with regret, one of two primary emotions involved in decision-making.

The feeling of disappointment is a adjustment, or reconciliation between fantasy and reality. Disappointment can only occur when your expectation of how things should be is different from how things really are.

How can I use disappointment?

Disappointment is not harmful but it indicates that you may have a tendency to create and attach to fantasies. Use it as a means of auditing and adjusting for that tendency.