When i feel dissappointed i usually go do something i like. but usually it depends on what im dissapointed in. For example if im dissapointed in myself. i usually try again for whatever i dissapointed myself for. If it was not getting a date with someone. I'll try again for something better. If i get a date with someone better. It makes me feel like it wasn't my fault for getting one the first time. Now if i was dissapointed in someone else. id have a talk with that person or if they won't talk to you then they have bigger problems. Usually doing something i love to do helps me get over my dissapointment.
Disappointment is nothing more than expectations not meeting your demands. I had to understand why I had such a high expectation level. Once I did understand this, I no-longer became disappointed and anything positive that cam about was a plus. If you don't expect it, it becomes a plus.