single word or phrase for particular feeling of release

Is there a word that describes when you have a great idea, revolutionary even, and then you find out it has already been done? Or maybe, that feeling - not futility, not Sisyphean - but that wry release of pent up motivation to act, upon finding out, there's nothing left to do.

Solution 1:

Deflated:

feeling less ​confident and ​positive than before; to reduce in size, importance, or effectiveness (Cambridge dictionary/MW)

An example of its usage:

But on that August afternoon when we finally landed at Narita and took our bearings, we felt deflated. The place looked depressingly ordinary. - NYT