Something disheartening makes you lose hope and the will to continue. This form describes the thing, the news, or the result — not the person who feels it. A disheartening score, a disheartening silence, a disheartening number of empty seats. Same slightly careful register as the rest of the family: common in writing and in serious talk, a little heavy for the playground. Though actually, people use it for small everyday letdowns too, not only for big life blows; a tiny turnout can be called disheartening without anyone claiming the world has ended. The important split is grammatical: you feel disheartened; the news is disheartening. English does this with many feeling words (bored / boring), and this pair works the same way.