You can only guess at what mental suffering they endure.

你只能猜测他们忍受着什么样的精神痛苦。

It caused her immense mental suffering.

这件事使她精神上极度痛苦。

At that time, she or he is in mental suffering.

那时，他(她)正承受着精神上的折磨。

This experience caused him much mental suffering.

这一经历给他的精神造成极大痛苦。

This experience caused him much mental suffering.

这一经历给他的精神造成极大的伤害。