I have a hard time feeling compassion for a person if I cannot see that person, interact with that person, see the pain in the person. So, how can I genuinely forgive someone who is not in my life at the present time?
You probably should start with thinking exercises. In the book, Forgiveness Is a Choice, I talk about taking wider perspectives on the one whom you want to forgive. These perspectives (personal, global, and cosmic perspectives) help you to see the other as vulnerable, as perhaps hurting, and this may lead to compassion for you.
For additional information, see What is Forgiveness?