(1) You want a word that means the inability to return to one's
good mood after some depressing incident.
(2) In my dictionaries and on the Web, I found a few words that
might meet your requirement.
(3) Let's say that I was an enthusiastic supporter of politician Ms. X.
Then I discover that Ms. X is no better than any other politician. I
would not be able to return to my previous mood of hope. So maybe
I could be described as:
(3) Let's see what others suggest.