You are correct, you have a cornish X meat bird. They usually don't live past butcher date. You have two options, 1 butcher her at 8 weeks of age, or once she reaches a desirable weight, or put her on a stick diet. 1/4 of 18% protein food daily until her weight drops to a healthy weight. You'd have to keep her in a separate pen were she didn't have access to any of the other birds feeders. If you tried this, there is still no guarantee that she will drop enough weight fast enough. Plus, being in her own pen by herself wont be and ideal situation for her, but it could still be worth a shot.
Whatever you comfortable with doing will be the right decision. If you want to try and save her, I have an article about my 2 pet Cornish X that may be helpful. Good luck!