Have you ever had a hard time doing something? Ever needed to lose weight, have you ever broken up with someone you didn't want to break up with and had a hell of a time staying out of contact with them? Ever failed at trying something. Do you ever wake up and think, something like, I'm going to get in to shape! I'll go for a run this afternoon and get started and then stayed in to watch tv instead? Become a vegan? Cut down on - whatever your comfort food is?
If not, this may not make any sense but it gives you an idea of the resolve you can have when you KNOW you should be doing something so you say, ok I will do it (and you mean it!) but when push comes to shove, it's just too hard.
These things give you a vague idea of why she says, I'm done! I'm going to quit! She may mean it but addiction is a serious thing, she is addicted physically and mentally and withdrawal can be hell (and also very dangerous) as dandylion has mentioned.
Don't expect her to beg you for help, it's unrealistic. The uphill battle, after the withdrawal must look like a mountain. Only a professional working with her could probably get to the bottom of that. She has to have the will to quit, nothing you have said indicates she is interested in doing that right now. It's her choice.