I don't want to talk to my mom about my dad and his new fiance
Why not discuss this with your parents.
They always talk bad about me.
While your parents should respect your thoughts, you should do the same. Be the bigger person.
I do not want to be the bigger person. I'm going to tell my mom that my dad doesn't love her.
That will not go well. It can be hard to get along with your parents, but if you don't respect them, it will be even harder. They can ground you and you will have it worse.