I understand your point that I can forgive a person who has died because forgiveness in this case is an internal transformation from resentment to thoughts that the other has inherent worth, with the accompanying compassion toward the person. Yet, what about a situation in which I want to seek forgiveness and the other is no longer among the living?
The answer depends on your belief system. If you are a secularist or atheist, you can go to the person’s family members if what happened affected more that the now-deceased person. You can describe what you did and ask them for their forgiveness. If you are a monotheistic believer, you can go to God and confess your transgression and ask for forgiveness. You need not keep the feeling of guilt in your own heart, but can experience relief.
For more information, see Faith and Religion.