How do you deal with bitter feelings? [I Need Advice]

I had a ex-friend who did me wrong. I don't want to get into the full story but this person degraded and threatened me. Today I've moved away from this person and no longer speak to them but I see pictures on facebook and hear about them. My past with this ex-friend keeps me up at night. I get angry. I feel bitter and resentful. They seem to be happy and enjoying life. There's a part of me that struggles to see them be okay when they never tried to apologize for what they did to me. How do I let go of this situation? I just want to forget about how I was treated but I have such deep, negative feelings towards them. I've thought about therapy but I don't know.