Forgiveness is one of those terribly difficult things in Christianity. Old Testament theology, and the concept of 'like for like' punishment seems fair to us…
But if there is serious injury, you are to take life for life, eye for eye, tooth for tooth, hand for hand, foot for foot, burn for burn, wound for wound, bruise for bruise. Exodus 21:23-25
Many systems of justice have been based on this principle, and still are, including in countries whose laws are, at least notionally, Christian. But I posit that they are not actually Christian (which implies a New Testament 'take' on justice), but 'Old Covenant' - based on the Law of Moses.
Christians are asked to walk a harder road - one which seems counter-intuitive to us humans. Somehow we seem to expect 'eye for eye' justice - as a sort of vengeance as much as anything. I often think to myself when someone is interviewed on the TV news, following a crime, and they say 'we want justice', that actually what they want is revenge - they want the offender to suffer at least as much as the person they injured.
As Christians, we are called to forgive. The verses in Matthew 6 which follow immediately after Jesus teaches the disciples to pray the 'Lord's Prayer' say:
For if you forgive other people when they sin against you, your heavenly Father will also forgive you. But if you do not forgive others their sins, your Father will not forgive your sins. Matthew 6:14-15
There is no caveat here - no sense of 'once they've been punished you should forgive them'; instead this command is an absolute - you must forgive them. And only then will God forgive you. That's hard, very hard. Hard to accept, and hard to do in practice - even too hard to do without help.
Hebrews 9:19-22 says:
When Moses had proclaimed every commandment of the law to all the people, he took the blood of calves, together with water, scarlet wool and branches of hyssop, and sprinkled the scroll and all the people. He said, "This is the blood of the covenant, which God has commanded you to keep." In the same way, he sprinkled with the blood both the tabernacle and everything used in its ceremonies. In fact, the law requires that nearly everything be cleansed with blood, and without the shedding of blood there is no forgiveness.
So there is still a requirement, under the New Covenant, for punishment/sacrifice for wrongs and to purify. But (and it's the biggest, boldest, 144-point text, BUT you can think of)…
The whole point of Jesus' death on the cross was to be the one, perfect, all-encompassing, sacrifice - God sacrificing Himself so as to restore the relationship between God and mankind. The book of Hebrews says:
...when Christ came into the world, he said:
“Sacrifice and offering you did not desire,
but a body you prepared for me;
with burnt offerings and sin offerings
you were not pleased.
Then I said, ‘Here I am—it is written about me in the scroll—
I have come to do your will, my God.’”
First he said, “Sacrifices and offerings, burnt offerings and sin offerings you did not desire, nor were you pleased with them”—though they were offered in accordance with the law. Then he said, “Here I am, I have come to do your will.” He sets aside the first to establish the second. And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all.
Day after day every priest stands and performs his religious duties; again and again he offers the same sacrifices, which can never take away sins. But when this priest {i.e. Jesus Christ} had offered for all time one sacrifice for sins, he sat down at the right hand of God, and since that time he waits for his enemies to be made his footstool. For by one sacrifice he has made perfect forever those who are being made holy. Hebrews 10:5-14
What we have to get our heads around is that there is no need for any more sacrificing, and absolutely no place for revenge or 'justice' - we must forgive, because Christ forgave us, and died for us - to pay the price for our wrong attitude - wrong towards each other, and wrong towards God.
So, and this is the difficult bit… There is no call for us to be seeking 'eye for eye' - any 'debt' we might imagine has already been paid in full, and we must acknowledge Christ's work in that. Once we realise that, then the forgiveness from Matthew 6:14-15 becomes easier to understand, and our gratitude to Jesus Christ should become all the greater, for His act sets us free to forgive.
I would almost go so far as to say that if we do not forgive, we are saying, in effect, that Christ's death on the cross was not the 'one, perfect, sacrifice for sin' (in the words of the Anglican communion liturgy) - that, in this case (the one we are not prepared to forgive), His death was not sufficient; therefore sacrifice was not perfect. If His sacrifice isn't perfect, then what right do we have to assert that our own sins are forgiven?
I was bullied, fairly severely, for most of my school career. On a human level, I would prefer that the perpetrators of that 'abuse' burn in hell-fire for ever. But I am commanded to forgive. I do not think I could ever do this, on my own - I have neither the desire nor the strength of will. It has been a decades-long struggle, to learn to forgive those people at all levels of my being. But I have, with the help of God, 'got there', I believe. And that is curiously liberating. I am no longer 'held back' by bitter memories, or resentment for the 'years the locusts have eaten' - instead I am at a point where God has restored to me those years - when I look back, I mostly see the good times instead of the bad (and there were some good times!); and as time passes, I remember more of the good things. Some of that has come about through my own kids - going places, and doing things, which I was taken to, and did, as a child myself. Children are a blessing, in more ways than we often realise.