Perhaps it's possible to forgive in one grand swoop, but I didn't experience it that way. I did it in bits and pieces. You forgive what you can, when you can. To forgive does not mean overlooking the offense and pretending it never happened. Forgiveness means releasing our rage and our need to retaliate, no longer dwelling on the offense, the offender, and the suffering, and rising to a higher love. It is an act of letting go so that we can go on.
If we just worry about the big picture, we are powerless. So my secret is to start right away doing whatever little work I can do. I try to give joy to one person in the morning, and remove the suffering of one person in the afternoon. If you and your friends do not despise the small work, a million people will remove a lot of suffering.