My girlfriend (21f) keeps on choosing her bestfriend (21f) who disrespected me (24m) and our relationship over me.

So my girlfriend has a best friend (21F) who in my opinion has always been jealous of her and has always hated seeing her succeed. She says the most vile shit ever known to mankind whenever she talks. A fight broke out between me and her last year, and she told me that my girlfriend had been cheating on me with her ex every time she was in his city. Later I was told she only said it to hurt me and that none of it was true. I still don't understand how my girlfriend's best friend could disrespect both my girlfriend and our relationship by saying something that disgusting. Eventually, they made me feel bad for questioning my girlfriend's loyalty and for believing those lies. I told my girlfriend to cut ties with her, but she keeps talking about her and wants to text her. I don't know what to do about this. I've expressed everything to my girlfriend very clearly, but she still wants to stay friends with her disgusting best friend. How do I handle this?