Do you think that the root cause of any person’s persistent anger, pessimism, and bitterness is unforgiveness?
I would not say that the root cause is unforgiveness. The root cause is deeply unfair treatment by others, acts of injustice. I would say that as people fail to find a solution to their initial shock and anger, then unforgiveness eventually does play a part in a person continuing to live with deep anger, pessimism, and bitterness. Forgiveness offers a way out of these consequences of being treated deeply unfairly.