If we choose not to forgive we are sinning just as the person who committed the offense is in sin. If we remain angry and offended we are sinning and we usually commit more sin by telling people about it, fuming about it, and holding a grudge about it. We are also giving the offender a position of power over us and we are in bondage to them. If we turn the anger inward, it is toxic waste and will make us sick one way or another - physically, emotionally, spiritually or a combination of all 3. In prayer ministry I often meet people who are depressed because they have turned the anger inward. I also meet people who are angry over something that never happened.
I am not suggesting you tell the parents of a murdered child or a rape victim they have to forgive the day after the tragedy. I am not saying it is easy. I am saying with God's grace and only with His grace, it is possible and He tells us we must forgive. Forgiveness may come in an instant, it may be a process.
These are but a few verses from the Bible dealing with forgiveness:
if my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then will I hear from heaven and will forgive their sin and will heal their land. (This does not really pertain to forgiveness between 2 people, but I love the verse - it's called blogger's discretion.)
For if you forgive men when they sin against you, your heavenly Father will also forgive you.
But if you do not forgive men their sins, your Father will not forgive your sins.
Then Peter came to Jesus and asked, "Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?" Jesus answered, "I tell you, not seven times, but seventy-seven times. (Not the answer Peter expected. I so identify with Peter and his mouth.)
Do not judge, and you will not be judged. Do not condemn, and you will not be condemned. Forgive, and you will be forgiven.
If he sins against you seven times in a day, and seven times comes back to you and says, 'I repent,' forgive him."
Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you.
Above all, love each other deeply, because love covers over a multitude of sins. (Love makes it easier to walk in forgiveness and die to self.)