Best friend?

I don't know how to explain this but I'm friends with a girl who I thought was my best friend. We have been friends for almost 3 years. She's started treating me really badly lately since she moved out of her parents home and I'm still living with my mum. I have always been there for her. When ever we make plans she doesn't show up at last minute but she doesn't tell me first, I have to always text her and she comes back with "oh I can't sorry blah blah blah happened". I feel like I'm the one she goes to when no one else is there and I'm sick of her walking all over me. She puts everyone else first but still calls me her best friend. What do I do?