Rather, be kind and compassionate to one another, and forgive each other, just as God forgave you in Christ. Ephesians 4:32
Forgive
Anyone who has had a relationship knows what it feels like to be forgiven and forgive someone, but also what it feels like when someone does not forgive you. I can tell you from my own experience that not to forgive is destructive. The lack of forgiveness weighs, it's as if you were carrying a dead person on your back.
Every marriage will have difficulties to survive if the spouses do not forgive each other. If you think it is okay to live without forgiving someone or not being forgiven, try living with your spouse or working for your boss that way. It's going to be a tense and horrible time. In some cases, there are people who believe that even God can not forgive them, but I ask them:
Do not you think Jesus' payment on the cross was enough? Or, if you can not forgive others, I ask you: Do you realize that Jesus died for us while we were still sinners, and enemies of God?
God has forgiven us infinitely more than we could forgive others in a million lives. So we must forgive, and we must also learn to accept forgiveness, even from God. Unforgiveness binds us to the past and impacts our future.
Forgiving
If you want a great description about forgiveness, it is on the cross, and it is not in us. While Jesus was bleeding and dying in agony on the cross of Calvary, he prayed to the Father:
Father, forgive them, because they do not know what they are doing (Luke 6, 23.34). How could Jesus ask the Father to forgive those who were doing something so evil?
Jesus himself said: Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you (Luke 6: 27-28). That implies that we forgive those who hate us and persecute us.
God forgave us in Christ, although none of us deserved it (Romans 3: 10-12). It was only out of love (Eph 2: 4) that God was moved to act on our behalf, giving life to a corpse, held captive by the enemy (Eph 2: 1-2). There was nothing special about that. We were "children of wrath, as well as others" (Eph 2,3), but God loved us and did something for us (2 Tim 2:25, 1 John 4:19).
Let me ask you: How many of our sins were before us on the cross?
The answer is: Everyone! God forgave us in Christ, so we must forgive others in the same way. Forgiveness was given in love and we must act in love by forgiving others. Jesus orders us to ask for forgiveness and pray to God to "forgive us our debts, as we forgive our debtors" (Mt 6,12).