Tell someone, but make sure you don't publically embarrass her. Rumors spread and high school is a pain already. You don't need to have that shit spread and affect the both of you.

Once you're done telling her parents (preferably) or a counselor, break up with her. Shes manipulating you which is abusive. You are not personally responsible for someones happiness. While it's best to have sympathy for her, and inform an adult that can help her so she can get help, you do not have to be the one to stick with her for the rest of your life.

I suggest giving this a watch:
https://youtu.be/4H9jTQKmR3Q