Nothing in this article is meant to be medical advice. Please consult your own healthcare provider.
Some info below taken from:
www.apa.org/monitor/2017/01/ce-corner.aspx
Forgiveness and Health, Toussaint, Worthington and David R. Williams, PhD, editors
www.ncbi.nlm.nih.gov/pubmed/20194301
https://greatergood.berkeley.edu/topic/forgiveness/definition
https://greatergood.berkeley.edu/article/item/nine_steps_to_forgiveness
https://greatergood.berkeley.edu/article/item/truth_and_reconciliation/
Beyond Revenge: The Evolution of the Forgiveness Instinct. Michael E. McCullough
https://greatergood.berkeley.edu/article/item/the_ancient_heart_of_forgiveness

Last month, we looked at the healing benefits of gratitude. Let’s look together at the healing benefits of forgiveness. U.C. Berkeley’s Greater Good Science Center is a rich resource. See links above.
What is forgiveness?
Science has demonstrated that both revenge and forgiving are hard-wired into humans (and other primates). 

“Psychologists generally define forgiveness as a conscious, deliberate decision to release feelings of resentment or vengeance toward a person or group who has harmed you, regardless of whether they actually deserve your forgiveness.” (https://greatergood.berkeley.edu/topic/forgiveness/definition)
Forgiving is not about letting the other off the hook, making excuses, condoning or glossing over the hurt. It is not even about forgetting.
In fact, forgiving can be more about a way to smooth one’s own recovery than a way to benefit the other. Others don’t even have to know that you have forgiven them, as some relationships are so toxic it is counter-productive, even dangerous, to engage at all.
“Though forgiveness can help repair a damaged relationship, it doesn’t obligate you to reconcile with the person who harmed you, or release them from legal accountability…
“A former victim of abuse shouldn’t reconcile with an abuser who remains potentially dangerous, for example. But the victim can still come to a place of empathy and understanding.” (https://greatergood.berkeley.edu/topic/forgiveness/definition)
Bishop Desmond Tutu of South Africa states, “If the victim could forgive only when the culprit confessed, then the victim would be locked into the culprit’s whim, locked into victimhood, no matter her own attitude or intention. That would be palpably unjust.” The victim remains shackled. “Forgiveness gives us the capacity to make a new start.”
How does forgiving heal?
Research has shown that forgiveness is linked to positive mental health outcomes such as reduced anxiety, depression and major psychiatric disorders. Benefits include improved relationships and a greater sense of connectedness to others.
Other research has shown that social connectedness is essential for trauma healing. Forgiving is associated with fewer physical health symptoms and lower mortality rates. By reducing stress and eliminating toxic anger, forgiveness lowers blood pressure and prevents coronary heart disease.
Learning the skills of forgiving: Please see the links above for several approaches to cultivating your ability to forgive. My favorite is https://greatergood.berkeley.edu/article/item/the_ancient_heart_of_forgiveness by Jack Kornfield:
“Discover that it is not necessary to be loyal to your suffering. …We are so loyal to our suffering, focusing on the trauma and the betrayal of ‘what happened to me.’ OK, it happened. It was horrible. But is that what defines you? …
“The Dalai Lama, whose culture in Tibet has been destroyed, has stated, ‘… They have taken so much. Why should I also let them take my joy and peace of mind?’”