The closest Japanese word to がっかり (gakkari) is probably 失望 (shitsubou, disappointment). Both express a sense of letdown, but the strength and specific flavor differ. Gakkari describes milder disappointment, often tinged with a degree of resignation. It's the feeling when something doesn't quite meet your expectations, but you're not deeply devastated. Shitsubou, by contrast, implies a more profound sense of loss or hopelessness.

Consider a scenario: Haruki has spent weeks preparing for a school festival performance. He's rehearsed his lines, crafted his costume and is really looking forward to it.

Gakkari might apply if, on the day, his performance is alright, but the crowd's reaction is muted, and his friends' praise seems half-hearted. He feels a bit deflated. Things didn't go as well as he'd hoped, but it's not a complete disaster. He might say, “今日の劇、ちょっとがっかりだった (Kyou no geki, chotto gakkari datta) --- The play today was a little disappointing.” He's not crushed; he just wishes it had been better.

Now, let's change things up using two more related words: 落胆 (rakutan, discouragement) and 意気消沈 (ikishoushin, dispirited). Rakutan is similar to gakkari in that it also involves disappointment, but it leans toward a sense of loss of confidence following an unsuccessful project or ambition. Ikishoushin refers to a state of being dejected and losing morale, usually after failing to accomplish something.

If Haruki had completely flubbed his lines, messed up his costume, and the audience was openly laughing, he may feel rakutan. He may feel like he's lost his confidence. If he has been looking forward to this performance for a long time, but it ends in total failure, and he can't stop thinking about what a failure he is, he would feel ikishoushin.

If instead, two classmates he admires told him the performance was amazing, he might not feel rakutan or ikishoushin, but he might still feel gakkari when comparing it to his own personal hopes for the performance.

In short: Choose gakkari when your hopes took a small hit. Don’t use it for true heartbreak (shitsubou), loss of confidence (rakutan) or feelings of dispiritedness (ikishoushin).