What should I do?

I'm thinking about saying goodbye for ever to my best friend in the world, but I don't know if I should or not. She's been there for me since we met, and I've been there for her too. Our lives are completely the same. But she's changing for the worst, and she's startin to make horrible mistakes left and right. I can't stop her and I'm tired of her getting hurt. When she makes horrible mistakes, it hurts me. I don't want to hurt her, but I don't want to live in pain. I really confused. Help please!