Backstabbed?

I had this best friend that I’ve known for 4 years. Ever since I dated this one dude back in 7th grade, she had a thing with him and she knows he likes her right now. She knows how I feel about him and the whole ex thing. We recently had a huge fight about it. And then she goes on about how it’s wrong that these other people did the same thing to their ex best friend. She’s a huge hypocrite and she’s very negative about everything. She told me I’m going to have to deal with how she is because she ‘can’t change’. Is it worth to keep someone around like this? Is it wrong to be mad at her for basically keeping my ex around? And, she wanted to take him to military ball when me and her were going. This probably sounds childish. Sincere responses please :/