“He is the sinner.”
If a person refuses to forgive someone who has wronged him, there are two ways to view that non-forgiver.
One way is to just forget about him. You tried to appease him, and he was unwilling. Walk away.
Another way, more ontological, is that besides for the law that you don’t have to bother anymore, the Rambam adds that the non-forgiver is the sinner.
Not “a sinner” but “the sinner”.
What does that mean?
It means that if you hurt someone, you are the sinner, the one who is responsible for that relationship being broken.
Now, if any actual financial or material restitution has been made, and all that is left is the subjective emotional pain,
and you have sincerely apologized and asked forgiveness three times, and the offended party refuses to forgive, he is “the sinner”, he is the one who now bears the responsibility for that relationship being broken.