dad won't listen

It’’s not fair to place him in a position where he has to choose between you or her. Chances are she’s asking to come along to try and get to know you. Yes, it’s a relationship that spawned from adultery. Yes, you have a right to not want anything to do with her. But at the same time you have to decide for yourself if you’re really going to give her the satisfaction of basically capitalizing your father’s time.

You can either suck it up and spend time with them both (she may not be that bad of a person), or you can confront them both and ask if every once and a while you and your dad could do things on your own. When you say, “I won’t hang out with you if she’s there.” that just sounds too demanding. You need to compromise. It sucks, but that’s how it has to be right now. I’ve been in a similar situation myself with someone in my family.

Now, if they won’t compromise with you, ask your mother to talk to your dad for you. I’m assuming your mother has custody of you (if they’re divorced) and your father gets visitation, which means she has the power to set terms. Talk to your mom and tell her your dad’s girlfriend seems to be invading your space and you talked to her and your father, and they won’t reason with you.

You should approach this as logically and rationally as possible.