I want to tell my girlfriend to get her own copy of Animal Crossing
That costs a lot. Why can't she just use the copy you have?
Because she never returns the games she borrows.
I get where you're coming from with past experiences with your girlfriend and her not returning games that she borrows. I think it would be best having a conversation with her about this instead of outright refusing her.
I don't want to be someones bitch yo. She is not paying any bills, and I'm getting a bit tired of it.
She's your girlfriend though, she may just be having a rough patch in her life and letting her borrow your game can mean a lot to her as she patches up! How about talking about it while she plays, so you can find out why she's not pulling her weight recently.