I don't want to meet my moms boyfriend who was part of the reason my parents separated
I understand why you would feel this way, do you not like him?
I hate him for mistreating my mom
Even though your mom's boyfriend mistreated her, being resentful and avoiding him isn't correct. Harboring anger within yourself will slowly eat away at you.
But I hate him. I don't want to be in the same room as him, we can never resolve our issues.
I think that you should try to be more forgiving. It is not good to be that bitter towards your parents.