There were so many nights when I, as a young boy, had to watch helplessly as my father verbally and physically abused my mother. I can still recall the smell of alcohol, see the fear in my mother's eyes and feel the hopeless despair that comes when we see people we love hurting each other in incomprehensible ways. I would not wish that experience on anyone, especially not a child.

If I dwell on those memories, I can feel myself wanting to hurt my father back, in the same ways he hurt my mother, and in ways of which I was incapable as a small boy. I see my mother's face and I see this gentle human being whom I loved so very much and who did nothing to deserve the pain inflicted on her.

When I recall this story, I realise how difficult the process of forgiving truly is. Intellectually, I know my father caused pain because he himself was in pain. Spiritually, I know my faith tells me my father deserves to be forgiven as God forgives us all. But it is still difficult. The traumas we have witnessed or experienced live on in our memories. Even years later they can cause us fresh pain each time we recall them.

If I traded lives with my father, if I had experienced the stresses and pressures my father faced, if I had to bear the burdens he bore, would I have behaved as he did? I do not know. I hope I would have been different, but I do not know.

My father has long since died, but if I could speak to him today, I would want to tell him that I had forgiven him. What would I say to him? I would begin by thanking him for all the wonderful things he did for me as my father, but then I would tell him that there was this one thing that hurt me very much. I would tell him how what he did to my mother affected me, how it pained me.

Perhaps he would hear me out; perhaps he would not. But still I would forgive him.

Why would I do such a thing? I know it is the only way to heal the pain in my boyhood heart. Forgiveness is not dependent on the actions of others. Yes, it is certainly easier to offer forgiveness when the perpetrator expresses remorse and offers some sort of reparation or restitution. Then, you can feel as if you have been paid back in some way. You can say: "I am willing to forgive you for stealing my pen, and after you give me my pen back, I shall forgive you." This is the most familiar pattern of forgiveness. We don't forgive to help the other person. We don't forgive for others. We forgive for ourselves. Forgiveness, in other words, is the best form of self-interest.

Forgiveness takes practice, honesty, open-mindedness and a willingness (even if it is a weary willingness) to try. It isn't easy. Perhaps you have already tried to forgive someone and just couldn't do it. Perhaps you have forgiven and the person did not show remorse or change his or her behaviour or own up to his or her offences – and you find yourself unforgiving all over again. It is perfectly normal to want to hurt back when you have been hurt. But hurting back rarely satisfies. We think it will, but it doesn't. If I slap you after you slap me, it does not lessen the sting I feel on my own face, nor does it diminish my sadness over the fact that you have struck me. Retaliation gives, at best, only momentary respite from our pain. The only way to experience healing and peace is to forgive. Until we can forgive, we remain locked in our pain and locked out of the possibility of experiencing healing and freedom, locked out of the possibility of being at peace.

Desmond Tutu and his wife, Leah, and their children, from left: Trevor Thamsanqa, Thandeka Theresa, Nontombi Naomi and Mpho Andrea, England, c1964. (c) Mpilo Foundation Archives, courtesy Tutu family

As a father myself, raising children has sometimes felt like training for a forgiveness marathon. Like other parents, my wife, Leah, and I could create a whole catalogue of the failures and irritations our children have served up. As infants, their loud squalls disturbed our slumber. Even as one or the other of us stumbled out of bed, the irritation at being woken and the thoughts of the fatigue that would lie like a pall over the coming day gave way to the simple acknowledgment that this was a baby. This is what babies do. The loving parent slides easily into the place of acceptance, even gratitude, for the helpless bundle of tears. Toddler tantrums might provoke an answering anger in a mother or father, but it will be quickly replaced by the understanding that a little person does not yet have the language to express the flood of feelings contained in his or her body. Acceptance comes.

As our own children grew, they found new (and remarkably creative) ways of testing our patience, our resolve and our rules and limits. We learned time and again to turn their transgressions into teaching moments. But mostly we learned to forgive them over and over again, and fold them back into our embrace. We know our children are so much more than the sum of everything they have done wrong. Their stories are more than rehearsals of their repeated need for forgiveness. We know that even the things they did wrong were opportunities for us to teach them to be citizens of the world. We have been able to forgive them because we have known their humanity. We have seen the good in them.

Desmond Tutu with Nelson Mandela in South Africa, 1998. Photograph: AFP/Getty Images

In the 1960s, South Africa was in the fierce grip of apartheid. When the Bantu Education system of inferior education for black children was instituted by the government, Leah and I left the teaching profession in protest. We vowed we would do all in our power to ensure our children were never subjected to the brain-washing that passed for education in South Africa. Instead, we enrolled our children in schools in neighbouring Swaziland. Six times each year we made the 3,000-mile drive from Alice in the Eastern Cape to my parents' home in Krugersdorp. After spending the night with them, we would drive five hours to Swaziland, drop off or pick up the children at their schools and drive back to Krugersdorp to rest before the long drive home. There were no hotels or inns that would accommodate black guests at any price.

During one of those trips, my father said he wanted to talk. I was exhausted. We were halfway home and had driven 10 hours to drop the children at school. Sleep beckoned. We still had another 15-hour drive back to our home in Alice. Driving through the Karoo – that vast expanse of semi-desert in the middle of South Africa – was always trying. I told my father I was tired and had a headache. "We'll talk tomorrow, in the morning," I said. We headed to Leah's mother's home half an hour away. The next morning, my niece came to wake us with the news: my father was dead.

I was grief-stricken. I loved my father very much and while his temper pained me greatly, there was so much about him that was loving, wise and witty. And then there was the guilt. With his sudden death I would never be able to hear what he had wanted to say. Was there some great stone on his heart that he had wanted to remove? Might he have wanted to apologise for the abuse he had inflicted on my mother when I was a boy? I will never know. It has taken me many, many years to forgive myself for my insensitivity, for not honouring my father one last time with the few moments he wanted to share with me. Honestly, the guilt still stings.

When I reflect back across the years to his drunken tirades, I realise now that it was not just with him that I was angry. I was angry with myself. Cowering in fear as a boy, I had not been able to stand up to my father or protect my mother. So many years later, I realise that I not only have to forgive my father, I have to forgive myself.

A human life is a great mixture of goodness, beauty, cruelty, heartbreak, indifference, love and so much more. All of us share the core qualities of our human nature and so sometimes we are generous and sometimes selfish. Sometimes we are thoughtful and other times thoughtless; sometimes we are kind and sometimes cruel. This is not a belief. This is a fact.

No one is born a liar or a rapist or a terrorist. No one is born full of hatred. No one is born full of violence. No one is born in any less glory or goodness than you or me. But on any given day, in any given situation, in any painful life experience, this glory and goodness can be forgotten, obscured or lost. We can easily be hurt and broken, and it is good to remember that we can just as easily be the ones who have done the hurting and the breaking.

The simple truth is, we all make mistakes, and we all need forgiveness. There is no magic wand we can wave to go back in time and change what has happened or undo the harm that has been done, but we can do everything in our power to set right what has been made wrong. We can endeavour to make sure the harm never happens again.

There are times when all of us have been thoughtless, selfish or cruel. But no act is unforgivable; no person is beyond redemption. Yet, it is not easy to admit one's wrongdoing and ask for forgiveness. "I am sorry" are perhaps the three hardest words to say. We can come up with all manner of justifications to excuse what we have done. When we are willing to let down our defences and look honestly at our actions, we find there is a great freedom in asking for forgiveness and great strength in admitting the wrong. It is how we free ourselves from our past errors. It is how we are able to move forward into our future, unfettered by the mistakes we have made.