Describing an emotion such as disappointment is an undertaking well served by describing situations which would cause this emotion, that is by conveying an archetypal scenario. Thus what follows.
My math professor had mentioned multiple times in class that an upcoming assignment could be done by way of spreadsheets or with a computer program. Given that I'm supposed to be a computer programmer, I figured that despite the bit of extra work involved I was either obliged or compelled to write the computer program. Hoping to not be merely a person who could write a computer program but instead perhaps someone who could (possibly, maybe) be described as having a talent for writing these programs, I spent some time and effort more than reason would suggest to construct a beautiful program, replete with color graphs and readable formatting. I went to some length to make it work with just a single double click, dissatisfied with the alternative prospect of explaining to my instructor how to enter a command into the shell, much less how to add java to the PATH. I added an equation parser so that input like sin(e^(ln(y-sqrt(x-y)))) could be interpreted correctly, a scrollable output window for tables of values, and 2 windows for 12 full graphs, labeled unambiguously with their associated equations. I even tested it on three different platforms to make certain it would Just Work (note it won't run on OSX because it doesn't have the latest JVM, Doesn't Just Work). So it was with pride and satisfaction that I sent it off along with the source code, wondering, wishing I could see the reaction.
A day later, I received the response. It was a request, for me to bring in a paper copy of my results; my professor, for whatever reason, didn't want to run my program.
That vignette could be used to describe disappointment, but with a catch: it would best illustrate the emotion only if the emotion was stated beforehand. If I had said that I was about to describe anger, frustration, or even success and satisfaction, would the story have illustrated the emotion any less? It is easy to see that a person could be angry in such a situation, frustrated too. But I could have just as easily felt flattered, were I to think that I had created a program so sophisticated it was to be handled with caution. If any of these clearly distinct emotions could have occurred, then circumstantial emotion is itself in some ways ambiguous (I hope that's not news). The ambiguity of emotion is something easily used to our advantage; it's a fact that when you feel positive emotions, you feel better, you're physically healthier. That being the case, why not exploit the ambiguity of emotion? In writing that program I learned a lot, I improved my life regardless of who sees or approves of it. I could feel disappointed, or I could feel a strong sense of self satisfaction in my accomplishment. Given those options the choice isn't very difficult! The power is in the fact that there is a choice, that you have a say in how you feel. You can spend all your time looking for reasons to be sad or you can spend all your time looking for reasons to be happy--either way you shouldn't be surprised by the results.