Why Forgive?

A dictionary definition: "Forgive" - to cease to feel resentment against (an offender) : 

forgive one's enemies.

The Nature of Forgiveness
Most psychologists recommend mustering up genuine compassion for those who have wronged us and moving on from the past, instead of allowing bitterness and anger to perturb our emotional well-being.
It is critical to remember that forgiveness doesn’t automatically mean a reconciliation. We don’t have to return to the same relationship or accept the same harmful behaviors from someone who has hurt us.
Although burying the hatchet usually brings peace to the soul, there may be some exceptions to that advice, such as a case of sexual abuse. Sometimes a victim becomes more empowered when they give themselves permission not to forgive.
Equally, and perhaps more important, is learning to acknowledge your missteps and forgive yourself. Self-forgiveness is often the first step toward a more loving and positive relationship with yourself, and therefore with others.(Psychology Today-https://www.psychologytoday.com/us/basics/forgiveness)

Bitterness and anger eat at our souls, disturbing any inner peace. What does it cost to stay angry and bitter? It turns us into negative people whose negative thinking eventually colors everything in our lives. You probably people like that.  They are not mentally healthy individuals.

Some won't forgive because they think that it means they most return to the same relationship or accept the same hurtful behaviors from some one who did the hurting. That is so wrong. It is poor self-care to allow someone to continue treating you badly. Do not accept that behavior from others, or even from yourself. Letting go of bitterness or anger is freeing yourself from that negative attachment.

Self forgiveness may be the hardest part, if you have been blaming yourself for other people's behavior. When you can let go of that self-blame you can be more yourself. You can find inner peace from forgiving.

Let it go. Let go of the anger and free yourself to be the best you possible.