In a colloquial way here in Spain (don't think they use it in Latin America) we use the word "chasco" to describe something that was very promising but ended up being disappointing. Eg: - Conocí a una chica en internet el otro día, ¡era guapísima! Quedamos para tomar un café el sábado, pero al final... ¡no era nada guapa! - Vaya, ¡qué chasco! (I met a girl on the internet the other day, she was so beautiful! We met for a coffee on Saturday, but in the end... she wasn't beautiful at all!) I don't know if it helps but I felt like sharing it :)

Podrías decir "desencanto". Hope it helps! :)