You have no obligation to forgive your mother's boyfriend. But you are hurting yourself by obsessing over him. That makes me think there is a lack of things going on in your life that can compete for your interest against this dismal obsession.

You are physically independent, which is a great start. Next, someone, other than your mother needs to become emotionally important to you. Work on friendships. That takes extra effort, if you tend to be sensitive and not real outgoing. (I know that first hand.) But it's the only way to build a meaningful life.

For someone like yourself, therapy may actually be somewhat counterproductive. A therapist gives you one-on-one individual, undivided attention, like you got from Mum, when you were a kid. It may almost feel like "love." But, remember, this person is paid to patiently listen to you. Real relationships are not like that. They're kind of scarey. There is the fear that a person may get bored talking with you and go away. A therapist won't do that. We can get addicted to the safety of relating to a therapist and feel other connections are less meaningful. Somehow we have to accept that risking rejection is a part of living a normal and good life.

What your mother said about wanting a tough boyfriend to beat you up is very disturbing. I hope you realize that your mother has serious psychological issues. She lacks maturity. She has gravitated toward a guy who sounds like an arsewhole. That's bad judgement on her part. She must be a rather needy, dependent person. But she is with this guy voluntarily, which is her right. He sounds like someone you'ld do well to keep away from.

At times you will be around him, like when you visit your mom. Work with your therapist on developing tactics for enforcing your legitimate boundaries. Sometimes, when others become rude and demeaning, you have to disengage and withdraw from their presence. You don't have to explain why to them either. Do it consistently, and they'll figure out where those boundaries are.