Crusader Kings III

This achievement can easily be unlocked playing as any character. To suffer from a mental break you will need to reach 100 stress. The bar that keeps track of your stress level can be found to the left of the character portrait. Also, a mental break can only happen once every 5 years.

Stress can be gained from various things, such as decisions that you make, the death of a family member or a friend or playing against the personality of your ruler. Furthermore, you want to avoid the decisions that lower stress. Also events like feasts or a hunt will lower the stress of your character, so it would be best to avoid those.

EDIT: Davefrendy commented a helpful tip in the comments that you can immediately reset your lifestyle perks at the beginning of your game to suffer from a mental break.