My dad is like that, and my mother is still waiting on him hand and foot, and in my opinion, has long since become completely codependent.
Let me tell you this...
1. You are a wonderful daughter and person. Proof - giving a shit enough to try to find help. Repeatedly.
2. You tried your damnedest, but she ain't willing to be open to one iota of advice, not ever. Believe that.
3. YOU DESERVE YOUR OWN LIFE AT THIS POINT. WE ALL ARE SUPPOSEDLY ADULTS - TREAT HER LIKE ONE AND LEAVE HER TO THE CONSEQUENCES OF HER OWN DECISIONS.
4. You won't be her favorite person at all for excusing yourself from that mind game, on which I am sure she depends, so be prepared for the possibility of some hurricane force shit when you bail.
5. You will still be a wonderful daughter and person after you refuse to get sucked in.
6. You will absolutely, definitely, positively feel so much lighter and freer and happier after you do this, and you DO deserve to be free from this worry.
I wish you all the best in this. Just a thought - if you do not do this now the situation will only be so much worse for you if you have hopes/plans to start your own family some day. She will just be the biggest pain in the ass baby of them all, when your time is then focused on your own children instead of her.
If you are in the mood to, just to see how she reacts, you could try to screw with her head some by just giving her exactly what she wants - the absolute shittiest foodstuffs you can source... Velveeta cheese, dollar store hot dogs, sugared and caffeinated soda, cheap cream-filled cookies, boxed Mac n'cheese... Not that this will "fix" her in the slightest, because only a highly experienced psychiatrist can do that (maybe), but you might gain some small insight about her.
Please, please, live your own life now. At least, try it - I really think you will find it so much healthier and happier.
I have a mantra that I have spouted for years... "If I eat right, I feel right. If I feel right, I exercise right. If I exercise right, I think right. If I think right, I eat right..." Phil-SC