Disillusion (v.): to make someone realize that something which they thought was true or good is not really true or good:
I hate to disillusion you, but I don't think she's coming back.

Disenchanted (adj.): disappointed with someone or something, and no longer believing that they are good:  
 By that time I was becoming disenchanted with the whole idea.

Letdown (n.): an event, performance, etc. that is not as good as you expected it to be:
The end of the book was a real letdown.

Anticlimax (n.): a situation or event that does not seem exciting because it happens after something that was much better:
Going back to work after a month travelling in China was bound to be an anticlimax.

Disillusion (v.): to make someone realize that something which they thought was true or good is not really true or good:
I hate to disillusion you, but I don't think she's coming back.

Disenchantment (n.) [uncountable]: disappointment with someone or something, and no longer believing that they are good:
  Voters expressed growing disenchantment with the government.

Letdown (n.): an event, performance, etc. that is not as good as you expected it to be:
The end of the book was a real letdown.

Anticlimax (n.): a situation or event that does not seem exciting because it happens after something that was much better:
Going back to work after a month travelling in China was bound to be an anticlimax.