[20F][50M][23F] how do I save my relationship with my dad when his girlfriend is coming between us ?

I’m 20 and my dad is 50 and we’ve always had a great relationship. But he’s has been dating this new girl who’s 23 for 1.5 year and ever since she came into his life everything has turned into a mess where my dad even threatened to cut me off. He just announced that he’s about to propose to her and that I have to start treating her a little better. I guess she told my dad that I’ve been rude to her, when all I do is not engage with her. I don’t care about her inviting me to dinner, or to hangout because I just don’t like her, and this is not me being rude.

Somehow she’s omitting part of the story where she’s bossing me around, telling me to do things like she’s my mom. This is so frustrating because whatever I say I say to my dad, I will be the bad guy. She’s clearly a gold digger, who’s using my dad but he can’t see it. Now she’s poisoning my relationship with my dad. At this point I don’t know what to do, I’m forced to be around this woman who’s literally stealing my dad from me. I’m out of way on how to deal with the situation, my dad doesn’t ask me how I feel about this relationship, he clearly doesn’t care about my feelings in all of this. I really love my dad but I fear this new woman is going to cost him our relationship.

What should I do about the situation? someone please help me figure this out.

TLDR:My dad’s child girlfriend is lying to him about me being rude.