General terms:
* Anguish: Intense mental or physical suffering.
* Agony: Extreme pain or suffering.
* Torment: Severe physical or mental suffering.
* Misery: Great unhappiness or suffering.
* Distress: Extreme anxiety, sorrow, or pain.
Words focused on mental suffering:
* Despair: Complete loss of hope.
* Grief: Deep sorrow, especially over a death.
* Heartbreak: Intense emotional pain, especially caused by a loss of love.
* Devastation: A state of complete destruction or ruin.
* Trauma: A deeply distressing or disturbing experience.
Words focused on physical pain:
* Excruciating: Extremely painful.
* Unbearable: Too painful to endure.
* Pangs: Sudden, sharp feelings of pain or distress.
The best word to use will depend on the specific context of what you are trying to describe.