To forgive is not to forget, but to re-examine in light of universal truth.
Okay, so some people do not believe in universal truth or any other absolutes or constants (well, possibly with the exception of the speed of light, or Planck's Constant, or Faraday's Constant, or Newton's Constant, or Avogadro's Constant…. okay, a number of scientific-community-approved constants).
So, then, how about this -- instead of absolute, universal, perfect truth, imagine a truth that is just relatively larger in its scope that any individual person's.
Now, multiply it by the number of possible persons, sentient entities and perspectives in the universe.
And there you have it, a finite truth, and yet about as "absolute" as it can get under the circumstances and with our limited homo sapiens brains to quantify and grasp it.
So, in the light of that Very Large Truth, to forgive often involves seeing the other side from another rather stereophonic perspective -- a whole bunch of perspectives, in fact -- and most of all, seeing from the perspective of the actual person who has wronged you.
In doing this, you reexamine the event and the action itself from all possible sides. No panel of judges can do it, only you and your suddenly "expanded" imaginary perspective.
And in light of such observations, you just might possibly come to understand why the person did what they've done to you, and why, and how, and all the myriad life decisions and reasons and cause-and-effect chains that made it all happen.
And after such close and complex scrutiny, there is a very good chance that you will be able to forgive.
Of course, even that's not a guarantee of forgiveness, but a possibility.
But it's better than nothing.