Is there a word or expression for when someone passionately wants something, and then discovers afterwards that it's different from how he imagined?

I would go with disillusionment.

A feeling of disappointment, akin to depression, arising from the realization that something is not what it was expected or believed to be, possibly accompanied by philosophical angst from having one's beliefs challenged.

[Wiktionary]

An example from vocabulary.com:

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.

A deep distinction between disillusionment and disappointment from the book "The Poetics of Disappointment: Wordsworth to Ashbery" By Quinney:

Buyer's Remorse

is the sense of regret after having made a purchase.

"[The] Bears may suffer 'buyer's remorse' over Cutler, but real leaders admit it" -Phil Rosenthal, Chicago Tribune

See also, Caveat emptor; Buyer Beware.

Even if it was the trip of a lifetime, you'd be lucky (though, irresponsible) to not feel a tinge of remorse at having spent your lifesaving's.

Cognitive dissonance

is the mental stress or discomfort experienced by an individual who holds two or more contradictory beliefs, ideas, or values at the same time, or is confronted by new information that conflicts with existing beliefs, ideas, or values.

Aw, man. I thought this was going to be easy.

"Disenchanted" is another one. It strikes me as somewhat less deep and abiding than "disillusioned", and may be a good fit for the OP's examples.

The proverb: "The grass is always greener on the other side (of the fence)".

Something may seem fantastic, but never as good as they appear from a distance. The expression particularly refers to wanting something that someone else has, or wanting to emulate a particular person.