Is there a word or expression to describe when someone passionately wants something, eventually gets it, and then discovers that it isn't what he thought it was?
For example, suppose that John is a high-school student who admires the football team. He dreams of becoming a football player, thinking of how popular and sexy he would be if only he could be on the team. Then, one day, he is able to join the team, and he immediately regrets his dream because playing football is so much harder than he could have imagined.
For another example, suppose that Kaylee has a life-long dream to visit some exotic land. After years of effort, she saves enough money and makes the right connections to make the trip. Upon her arrival, she is immediately devastated by how primitive the accommodations are and how she has no access to the comforts of life she had taken for granted at home.
Edit: Thanks for the great answers and suggestions. It seems that there are a number of different words and phrases for this, each with somewhat different meaning.