Has anyone ever done something so horrible to you that "I'm sorry" couldn't fix it?
Jesus said: "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." That is recorded in Matthew 6:14-15.
Forgiveness should come regardless of whether someone says "I'm sorry", or indeed even if the person is not sorry. It's not easy, because it is against human nature.
But that is part of why we need a new nature. The apostle Paul wrote: "Those controlled by the sinful nature cannot please God" (Romans 8:8). Indeed, describing life under the sinful nature, Paul said: "I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out" (Romans 7:18).
That is why Jesus told Nicodemus: "I tell you the truth, no one can see the kingdom of God unless he is born again" (John 3:3). Truly yielding to Jesus as our Lord brings a change of nature that is so profound that the Bible calls it being "born again". It is being born into a spiritual, rather than fleshly, life. Jesus said: "Flesh gives birth to flesh, but the Spirit gives birth to spirit" (John 3:6).
Only those who are God's children, who have been forgiven by Him through yielding to Him, can truly take to heart what Paul wrote to the Colossians: "Forgive as the Lord forgave you" (Colossians 3:13). It may be against human nature, but not against the spiritual nature that God imparts to us.
Of course, it is not always easy to yield to that nature, but we don't have a choice. God commands it. For us not to forgive is to disobey God and fall into sin. That means that the person most hurt by my lack of forgiveness is me!