You have to appreciate that your Ex once shared your life. You may have been married, you may have children. Getting from *&^& to polite is a hard step.
If your Ex was abusive, unfaithful, dishonest, etc. it is much harder to move to a normal relationship.
The point is not to perform for the public, the point is to lose any anger, emotion, connection.
You can’t do this when you constantly review their behaviour.
An abusive partner and an unfaithful one spur different feelings, as does one who stole from you.
You have to deal with your emotions until you don’t really care. Not just saying it, but really, not caring.