You can't help her if she isn't willing to help herself, yes it's a tough spot to be in but if someone is going to commit suicide no amount of love, help or caring you give them will stop it. I have been there with my sister and nothing we did helped.
Your last resort is to go for commitment, call the suicide hotline yourself. If she's in immediate danger they will put her on a psychiatric hold!
stay close, give her a hug and tell her your there to be with her, let her know how much you would be hurt if she was gone...
call mental health on her. One, it says you love her and want to help her and two stands your ground that you will not be held hostage by suicide threats.