From my understanding disappointment usually comes from an expectation not being fulfilled. So if we have disappointment it's important to look at our expectations. This may come into the eight worldly dhammas, what were we wanting that we were disappointed about? Were we wanting praise, gain, pleasure or fame? Did we get the opposite? Investigating into disappointment can help us understand a lot about ourselves and our craving.
When we have an expectation we need to open ourselves to disappointment. This is understanding craving and desire. It also helps to look into the third Characteristic of Existence, that many things are beyond our power to control. Sometimes we feel we can control the actions, the speech and thoughts of others, so we get disappointed when others do not behave as we would like them to. Or life is not as we would like it to be. So in order to let go of disappointment we have to open ourselves to the reality of what it is. And see the suffering that comes from craving.