General Antonyms:
* Dejected
* Downcast
* Disconsolate
* Despondent
* Mournful
* Gloomy
* Sad
* Unhappy
More Specific Antonyms:
* Grieving (for a specific loss)
* Anguished (intense emotional pain)
* Miserable (extreme unhappiness)
* Despairing (loss of hope)
The best antonym for "jubilant" will depend on the specific context in which it is used.