Sometimes things just aren't fair and have no justice. You have to suck it up. forgive and move on. No point in dwelling on it. But forgiving and the moving on is a process. It takes time and effort every single day.
Pray the forgiveness, submitting it to God. Allow God's peace to enter yourself as you submit the pain of whatever was done and the person's unawareness of the hurt caused.
You know, sometimes forgiveness isn't for the other person, but for yourself too. If you carry that hurt for too long it will end up eating you and hurting you. Once you forgive them for yourself you'll no longer feel that hurt or anger anymore because it no longer matter to you.