General synonyms:
* Apathy
* Lethargy
* Inertia
* Languor
* Enervation
* Lassitude
* Weariness
* Indolence
* Sluggishness
* Dullness
* Lack of enthusiasm
* Lack of interest
* Boredom
More specific synonyms:
* Depressed: If the listlessness is a symptom of depression.
* Exhausted: If the listlessness is due to physical or mental fatigue.
* Dispirited: If the listlessness is caused by loss of hope or confidence.
* Disillusioned: If the listlessness is caused by a loss of faith or belief.
* Uninspired: If the listlessness is caused by a lack of creative energy.
The best synonym for "listlessness" will depend on the specific context in which you are using it.