I'm going to tell my dad that I don't approve of his new girlfriend
Do you really dare to? What's wrong with your dad's new girlfriend?
I don't like her at all. She is an idiotic slut. I know for certain she's sleeping with other men.
Badmouthing your dad's girlfriend is disrespectful, you should be respectful to your dad and his girlfriend. You should hold your tongue and let your dad live his own life.
Fuck that, I'm going to tell him that she sucks.
Seeing your parent potentially replace the other is indeed hurtful, bu don't you think it's unfair to disrespect him? Don't you want your dad to find new happiness? How about trying to get to know his girlfriend, you might find her interesting!