I know the question a bit vague but that's the only plausible inquiry I could come up with. I wanted to what word can be used if someone is disappointed and they are surprised as to the fact that to start with they never thought the subject matter or the person which disappointed them would have such an effect on them.
Shell-shocked and Gobsmacked might be applicable to your context. A person is said to be shell-shocked when they're confused, shocked or anxious because of a difficult or unexpected situation and hence can't act/think normally. Gobsmacked is very similar where you're so surprised by an event that you lose the tendency to think normally or think at all.
"chagrin" comes to mind.
distress of mind caused by humiliation, disappointment, or failure. MW
- "To my chagrin, the party ended just as I arrived."
- "She had gained five pounds over the winter, much to her chagrin."
- "He decided to get a tattoo, to the chagrin of his parents."
You are disillusioned [with someone / something].
disappointed in someone or something that one discovers to be less good than one had believed.
I think the word you're looking for is nonplussed. From Oxford Dictionaries Online:
non·plussed nänˈpləst adjective
(of a person) surprised and confused so much that they are unsure how to react. "he would be completely nonplussed and embarrassed at the idea"
North American informal (of a person) not disconcerted; unperturbed.