This forgiveness meditation comes from the book: The Wise Heart – Buddhist Psychology For The West, written by Jack Kornfield. You can find it there from page 349 to 351. You can find a version also on his website.

Forgiveness enables us to be released from the sorrows of the past. Although it can arise spontaneously, it can also be developed. Before you can do forgiveness practice, you must be clear about what forgiveness means. Forgiveness does not in any way justify or condone harmful actions. While you forgive, you may also say, “Never again will I knowingly allow this to happen.” You can resolve to prevent further harm. Forgiveness does not mean you have to seek out or speak to those who caused you harm. You may choose never to see them again.

Forgiveness is an act of the heart, a movement to let go of the pain, the resentment, the outrage that you have carried as a burden for so long. It is an easing of your own heart. We have all been harmed, just as we have at times harmed ourselves and others.

For most people forgiveness is a process. When you have been deeply wounded, the work of forgiveness can take years. It will go through many stages—grief, rage, sorrow, fear, and confusion—and in the end, if you let yourself feel the pain you carry, it will come as a relief, as a release for your heart. You will see that forgiveness is fundamentally for your own sake, a way to carry the pain of the past no longer. The fate of the person who harmed you, whether they be alive or dead, does not matter nearly as much as what you carry in your heart. And if the forgiveness is for yourself, for your own guilt, for the harm you’ve done to yourself or to another, the process is the same. You will come to realize that you can carry it no longer.

To practise forgiveness meditation, let yourself sit comfortably. Allow your eyes to close and your breath to be natural and easy. Let your body and mind relax. Breathing gently into the area of your heart.

Let yourself feel all the barriers you have erected and the emotions that you have carried because you have not forgiven – not forgiven yourself, not forgiven others. Let yourself feel the pain of keeping your heart closed.

Then, breathing softly, begin asking and extending forgiveness, reciting the following words, letting the images and feelings that come up grow deeper as you repeat them.

Asking forgiveness of others

Recite: There are many ways that I have hurt and harmed others, betrayed or abandoned them, caused them suffering, knowingly or unknowingly, out of my pain, fear, anger, and confusion.

Let yourself remember and visualize the ways you have hurt others. See and feel the pain you have caused them out of your own fear and confusion. Feel your own sorrow and regret. Sense that finally you can release this burden and ask for forgiveness. Picture each memory that still burdens your heart.

And then to each person in your mind repeat: I ask for your forgiveness, I ask for your forgiveness.

Offering forgiveness to yourself

Recite: There are many ways that I have hurt and harmed myself. I have betrayed and abandoned myself many times through thought, word, or deed, knowingly and unknowingly.

Feel your own precious body and life. Let yourself see the ways you have hurt or harmed yourself. Picture them, remember them. Feel the sorrow you have carried from this and sense that you can release these burdens. Extend forgiveness for each of them, one by one.

Repeat to yourself: For the ways I have hurt myself through action or inaction, out of fear, pain, and confusion, I now extend a full and heartfelt forgiveness. I forgive myself, I forgive myself.

Offering forgiveness to those who have hurt or harmed you

Recite: There are many ways that I have been harmed by others, abused or abandoned, knowingly or unknowingly, in thought, word, or deed.

Let yourself picture and remember these many ways. Feel the sorrow you have carried from this past and sense that you can release this burden of pain by extending forgiveness whenever your heart is ready.

Now say to yourself: I now remember the many ways others have hurt or harmed me, wounded me, out of fear, pain, confusion, and anger. I have carried this pain in my heart too long. To the extent that I am ready, I offer you forgiveness. To those who have caused me harm, I offer my forgiveness, I forgive you.

Let yourself gently repeat these three directions for forgiveness until you feel a release in your heart. For some great pains you may not feel a release but only the burden and the anguish or anger you have held. Touch this softly. Be forgiving of yourself for not being ready to let go and move on. Forgiveness cannot be forced, it cannot be artificial. Simply continue the practice and let the words and images work gradually in their own way. In time you can make the forgiveness meditation a regular part of your life, letting go of the past and opening your heart to each new moment with a wise loving-kindness.

Source of the meditation:
https://jackkornfield.com/forgiveness-meditation/

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You can find several guided forgiveness meditations online also. Here is one of those:

Inner Peace – Outer Joy

Forgiveness meditation – ask for forgiveness of others, forgive yourself, forgive others

Asking forgiveness of others