We all can succumb to behaviors that are not reflective of who we really are.
Separating the action from the person is crucial to find some closure.

Most people who appear to take some pleasure in hurting you have some pain in their own lives. Most feel justified in what they are doing; otherwise, they would do something else.

Forgiveness is ultimately a form of empathy - it is taking whatever pain motivated the person and imagine that you have that same pain yourself. 

Resentment forces you to live in the past by fixing that person to that past moment.

Forgiveness is choosing to not let negative events of the past define how you feel.

Let go of the emotional attachment that you've developed. Let the hatred and anger dissolve, let the thoughts of revenge and misfortune die.

Once you've empathized with the person and decided they are also just human, ask yourself what role you want them to have in your life.

You can forgive someone and still maintain a boundary. They may not even necessarily know you forgave them.