I'm starting to have a problem

I have kicked my best friend to the curb, after I was through with her lying to me. I know everybody lies about one thing or another, I'm use to it. But I am not use to the way Sadie Riddle lies. She lies to hurt herself. I'm not sure how to handle that. She acts like she wants to change her ways and do whats right for herself at will make it so she can have a better life and makes promises to me that she is going to fix and clean up her life, but she doesn't. Instead she just goes behind my back and digs herself into a deeper hole. How can she do that to herself? How can anyone do that to themselves? I can't stand to be around people like that, they are toxic. I have looked out for her for years, but I'm done standing around and trying everything to help her to have her keep throwing my efforts back in my face, or so I thought. I keep letting her back in my life because I keep going back to believing her. I'm the stupid one, I know. I have lost count of how many times I have let her ruin everything. I have never been one to put up with shit but in the last 6 months that all I find myself doing. Maybe I'm the one who has the problem.