General synonyms:
* Depressed
* Dejected
* Downcast
* Disheartened
* Dispirited
* Hopeless
* Despondent
* Gloomy
* Low
More specific synonyms:
* Frustrated: Feeling annoyed or upset because of a difficulty or obstacle.
* Disappointed: Feeling let down or sad because something didn't meet your expectations.
* Deflated: Feeling drained of enthusiasm or confidence.
* Demotivated: Lacking motivation or enthusiasm.
* Resigned: Accepting something unpleasant without trying to change it.
Informal synonyms:
* Bummed out
* Down in the dumps
* Feeling blue
The best synonym will depend on the context of your sentence and the specific feeling you want to express.