You allowed yourself to be conned, again.
You allowed yourself to believe that there is something you could do to help her snap out of it, again.
Perhaps you still believe you can love her out of it. ( If only that were true, none of us would be here).
She is doing what addicts do and will treat people the way they allow themselves to be treated.
You can't control or cure her. You can however, control yourself and reactions. You are not her only option.
Next time, remember no is a complete sentance. No need to justify it cause then it turns into a negoitation. She knows what you want to hear and will play to it.
Take that cat food commercial song meow,meow meow meow and substitute no,no,no. no. Make it your personal mantra.
You are a terrific grandparent. That's more than good enough in anyone's book.