Your dad having a mistress is not ground for him to treat you bad. He should continue to treat you like his child regardless of his relationship condition with your mom. He is an asshole. You don't need to stoop to his level by trying to get even with him. Just leave him be.

Do you have other relatives? Like grandparents or uncles? People older than your dad will have better chance of talking some senses into him. You just do your parts as a child and take care of your mom. Don't tell her about your dad until she recovered. Don't stress her.