Let me make this clear. It is NOT your fault. You did your best, and something went wrong. That is life. If she is put down, it is the fault of her original owner, for dumping her at a high kill shelter. It is the fault of the shelter for not giving a gorgeous dog a chance, and for having rules that make it dang HARD to get her out of there. It is the fault of the shelter for not adopting a no-kill plan. It is the fault of her breeder for not keeping track of her. It is the fault, if you want to get broad, of society for creating and nuturing a model where the lives of companion animals are disposable, and of the ARistas for actively encouraging that model instead of actually fighting for the animals themselves. It is not YOUR fault.
Indeed, you were doing this out of the goodness of your heart. You've never seen that dog. You did not breed her, own her, she was never in your possession. She is only your responsibility, because you have mentally assumed that responsibility, because you are a good person who wants to help. There are people who owe her a great deal more than you, or anyone on Chaz, and they failed her. If she is put down, it is THEIR fault, not yours. YOU have done nothing wrong. YOU have done the best you could by her. YOU are still trying to do the best you can by her. That's a heck of a lot more than those who actually have a duty to her. So don't DARE blame yourself.