Unforgiveness comes with a high price. It binds the victim to the person who has hurt them. It also produces inner torment that can lead to sickness. People become what they focus on. It will destroy relationships, prevent healing, and cause the person to become self-destructive.
Resentment and unforgiveness gives place to the devil and allows him to build a stronghold on that ground in the believer.
Ephesians 4:27 NASB – do not give the devil an opportunity.