Feeling disillusionment means you're bummed out because you no longer believe in something — usually because you find out it isn't as amazing as you thought.
The definition of disillusionment doesn't sound so bad: being freed from false beliefs or illusions. Well, sorry to disillusion you, but it's not so great. Disillusionment is when the hard truth of reality makes you lose faith in your dreams and ideals. Like you might have thought your internship at the movie studio would be super glamorous, but you quickly suffered some disillusionment when you realized it involved 14-hour days and lots of coffee runs.