Marina Cantacuzino is telling me a story about two women, both of whom discovered that their partners were having affairs. For the first one – let’s call her Woman A – the infidelity, says Cantacuzino, “was seen as the act of ultimate betrayal, which not only ended her marriage but for the past 30 years has been the defining, obsessional, story of her life.”

As a result, her children, whom she had roped into the bitter, protracted war against her husband, more or less lost contact with their father.

Woman B was equally devastated to discover her husband’s affair, which came to light through text messages sent accidentally to one of their children.

But, over time, she decided that “These things happen, she was not going to let it ruin her life. The affair had to stop but it wasn’t going to be the big defining moment for her family.”

Some years later it was, says Cantacuzino, “as if it had never happened. There was no residue.”

Bitter marital breakdown. Estranged siblings. Adult children who “divorce” their parents. Friends who never speak again. Such all too common stories are a testament to the challenge of forgiveness in everyday life.

Extremes apart, almost everyone has a tale of an unthinking comment or unkind act (or several) by a family member or friend that can never quite be thrown off.

At the same time, none of us wants to be one of those slightly sad people with a long, knotted tale of deep resentment that is hauled out at every occasion, often over a string of seemingly minor incidents going back years: that person with a powerful investment in telling the story only one way – our way. Cantacuzino seems like an excellent adjudicator of such thorny issues. A former journalist with a calm, non-judging presence (she is a practising Buddhist), she set up the Forgiveness Project 10 years ago. A charitable enterprise, it has explored, largely through vivid storytelling, the many ways in which individuals deal with major trauma such as murder, sexual violence or acts of terrorism.

Her years as a journalist speaking to people about “the smaller injustices of their personal lives” meant that when she started collecting forgiveness stories she was ‘‘keen to avoid the ‘smaller’ and more personal narratives and concentrate instead on the more extreme stuff. I wanted to be more serious and engaged politically.”

The Forgiveness Project has done some remarkable work, bringing together perpetrators and victims from just about every serious war zone and area of conflict in the world, including Northern Ireland, South Africa and Israel. Sometimes it has ventured into controversial territory, such as its decision to hold a discussion at the House of Commons between Brighton bomber Pat Magee and Jo Berry, the daughter of Sir Anthony Berry who was killed in the IRA attack on the Grand hotel on 12 October 1984, during the Conservative party conference.

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The project has also gone into British prisons, bringing victims of violence face to face with those who have committed similarly extreme acts. One of the most absorbing and remarkable debates I ever attended was a Forgiveness Project discussion between the businessman Will Riley whose house had been burgled and Peter Woolf, the career criminal and heroin addict, who broke into his home that night.

Stories like these led Cantacuzino to fresh reflection on the difficulty – but vital importance – of thinking about forgiveness in ordinary life, and the charity is now broadening its work to look at more diverse, daily stories including those of medical mishaps and workplace conflict.

During our conversation, she mentions how much classic literature has the theme of forgiveness at its heart, from Middlemarch, in which George Eliot talks about “the hideous fettering of domestic hate”, to Anthony Trollope’s suitably entitled Can You Forgive Her?

Cantacuzino says, “Forgiveness is a nuanced thing. It’s a choice, a practice. In successful relationships, we’re probably doing it unconsciously on a daily basis. The English poet and philosopher David Whyte says, ‘All friendships of any length are based on a continued, mutual forgiveness.’ I love that quote!” She pauses, then laughs. “Or you could just call it, letting go of the rankle.”

Good word that, rankle.

So, I am curious to know, what makes the difference between a Woman A and Woman B? Is it age or experience – or are some things just hard-wired in our temperaments? (We often talk about a character as “proud” or “stubborn”, unsure whether to praise, pity or even decry such qualities.) And how much is it to do with the way we were brought up? Could we, should we, be raising forgiving children?

Styles of parenting obviously exert a powerful influence. “Clearly the attitude with which you rear your children is going to affect them, although each of them will be very different people. My mother was a good forgiver. She always said sorry to us when we were children, and I think that’s fantastically important. Similarly, a parent who never says sorry can create a child who finds it hard to do the same.”

Children can also learn empathy, an important part of the forgiveness process, from their early years. “It’s often about being able to put yourself in the other person’s shoes. A forgiving person would think: could I be that person? Did I perhaps play some part in this dynamic?”

Often a turning point for a victim of serious crime can occur with a face-to-face meeting with the perpetrator, coming to see them as a vulnerable human being, which can be hugely liberating. But, of course, in family life or friendship, we know the other person only too well (or we think we do) and this, paradoxically, can make it harder.

Everyone who has thought deeply about these issues agrees on one thing: there is no easy, quick path. Sometimes the choice to forgive, whatever that means, will come only after a lot of inner turbulence. Cantacuzino believes, “Feelings of rage and revenge come from pain, so it’s important to feel the pain, to grieve deeply for a loss. Some people call this process Hate, Hurt, Heal.”

But isn’t anger or even hate sometimes a legitimate response? Cantacuzino tells me a story about a father who left all his money to just one of his three adult children, leaving the other siblings furious and the relationships fractured. I protest at the unfairness and she says, “Life is messy. Perhaps we do best not to have expectations, to not think that we are owed everything, even by our family. It’s really not black and white.”

In this case, the story has a happy ending. The sibling who was left the money decided to share it out in an “effort to repair the relationship”. But I am left feeling the “rankle” on her siblings’ behalf and a nagging worry that too much emphasis on inner emotional worlds could, in the wrong hands, lead the less powerful to reconcile themselves to injustice rather than seek resolution of a wrong.

Cantacuzino is keen to stress, “Forgiveness is not about condoning or excusing bad behaviour. Forgiving is a tool for dealing with pain, a decision not to perpetuate the cycle. Reconciling with the past allows you to look to the future. The single thing that most characterises people who have experienced forgiveness is this larger perspective.”

The American journalist Harriet Brown has written about how, after years of obsessively hating her mother, she came to understand that while she might still choose not to meet her mother, she could make that decision in a different spirit. “Forgiveness, I begin to see, is not about pretending you don’t feel angry or hurt. It’s about responding out of kindness rather than rage. Even toward someone who’s hurt you deeply.”

Cantacuzino echoes this sentiment: “Sometimes it’s about acknowledging the hurt and choosing to let go. A few years ago, I had a friendship that went wrong. I tried to put it right, probably too hard. In the end, I had to realise, people have a right not to be your friend. And you not to be theirs.

“Sometimes we just have to give in to broken relationships. The writer Sharon Salzberg rather brilliantly calls it ‘loosening the grip of fixation’.”

• The Forgiveness Project by Marina Cantacuzino is published by Jessica Kingsley, £8.99. To order a copy for £7.19, go to bookshop.theguardian.com or call the Guardian Bookshop on 0330 333 6846