Resentment or grudges do no harm to the person against whom you hold these feelings but every day and every night of your life, they are eating at you.
You gave him an opportunity of showing greatness of character and he did not seize it. He will never forgive you for that.
I was angry with my friend: I told my wrath, my wrath did end. I was angry with my foe: I told it not, my wrath did grow.