General Distress:
* Upsetting
* Troubling
* Worrisome
* Agonizing
* Painful
* Unsettling
* Harrowing
* Grievous
* Heartbreaking
Emotional Distress:
* Depressing
* Anxiety-provoking
* Anguishing
* Heart-wrenching
* Soul-crushing
* Devastating
* Disheartening
* Discouraging
* Crushing
Physical Distress:
* Uncomfortable
* Painful
* Aching
* Sore
* Unbearable
* Agonizing
Mental Distress:
* Frightening
* Terrifying
* Horrifying
* Shocking
* Appalling
* Disgusting
Other:
* Inconvenient
* Burdensome
* Problematic
* Unpleasant
* Unfavorable
The best synonym for "distressing" will depend on the specific context and the type of distress you are trying to describe.