To gain peace of mind is to clear your mind of all grudges, hatred and bitterness! Forgiving all the time as Christ has forgiven us.
Life is too short to spend it with the weight of unforgiveness. "Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us," Hebrews 12:1
It is clear from the scripture that there are Sins and there are 'weights' that impede one's progress in life if adequate care is not taken. Unforgiveness is both a sin and a WEIGHT! You can't afford to carry such load in your journey in life.
The Offender and the Unforgiving suffer the same torture! Unforgiveness does not only bind the enemy but also the offended. You suffer more than your offender when you hold grudges. Bitterness is self torture and self destructive.
Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants. And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents. But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made. The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all. Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt.
But the same servant went out, and found one of his fellowservants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, Pay me that thou owest. And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all. And he would not: but went and cast him into prison, till he should pay the debt. So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done. Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me: Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee? And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him. 35 So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses. Matthew 18:23-35
In a nutshell, we are all offenders in one way or the other. We have no justifiable reason not to forgive those that offend us. No matter how grievous the offense may be.