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

Do not let yourself or the relationship be defined by anger. The ability to forgive and move on is critical for maintaining a healthy and happy relationship with the people you care about.

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.

Whatever this person has done, look for some explanation of their motivation. These are not excuses, just explanations. Before you forgive someone, it helps to understand the reason for their actions.

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

If forgiveness is the ability to see the person as a multi-faceted human being, empathizing with them is what gets you there.

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.

• Set rules. Define which behaviors you will and won't accept.
• Decide on consequences if someone breaks your rules.
• Communicate the above calmly.

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

We all do things in our lives that we regret, and then we hold on to shame and guilt. The process to forgive yourself is the same:

• Separate the action from yourself.
• Understand your motivation. Was it insecurity or ignorance that drove you to do this thing?
• Empathize with yourself. How much stuff do you blame yourself for that was not your fault?
• Mark your boundaries. 
• Eliminate emotional attachment. There are better things to do with your energy than hating yourself.