One of the hardest things I've had to deal with in life this far is to forgive others.
This doesn't come easy to do because once someone hurts you all you have is resent and anger towards them. But I noticed in myself that me staying in that dark, hard, and unpleasant place it has consumed my life. My thought processes and conscious thoughts were always triggered to "oh this person did this to me so I need to throw a pity party." But now I don't allow myself to do that for to long.
Don't allow what this person did to you control your life. They are winning. Don't let them win over you anymore. It is not worth it, it's not worth being unhappy for years over something that you cannot change anyway. Yes what they did to you was horrible, but forgiving them doesn't mean what they did was okay. Forgiving them is for you. You don't even have to tell the person that you forgive them. Forgiveness is for you. You are choosing to lift yourself out of the darkness, and not allow this person to have anymore control over you anymore.
You take control and you own your life. The past can never be changed, even though your mind is filled with loads of "what if's". We all have been there, we all have wanted to change an action or a situation. You are not alone. But you have all of the power within yourself to lift this burden off of you. This person cannot control you anymore, don't let them.
Forgiveness is a gradual process. It doesn't happen over night. But once you get to that point, you will feel free. Free as a bird flying in the sky. Take control of your life and sour into the sky. You got this, you can do this, don't give up. Let the light of forgiveness consume you.