For what it's worth, we had to make this decision in June for our 14yo black lab. Like you, I'd watched him fade for some time. I'd talked with my vet. My husband & I talked about it -- a lot and for months. How would we know? Could we (well, I) even do this? What was the right thing to do?
When it came down to it, I made the call. Our vet said he'd come to our house or we could come to his office -- whatever made sense for our family and would be easier on our old guy. And, honestly, as much as I loved that dog and miss him still, I was saddened by the need to make the choice, but totally comfortable with it because I knew it was the right thing for him and that, as someone who loved him, I owed him a comfortable, compassionate passing in my arms, with people who loved him right there helping him along. And once he was gone, I was relieved. Not because of the difficulty caring for him had been, but because he was free and, I believe, running like a puppy in his next life.
As I type this, I'm tearing up because even though we have 2 other dogs they aren't HIM -- and I miss my big black lug. But, still, I feel really good about how long we were able to keep him comfortable AND that, once I couldn't be sure he was OK and it seemed his good times were only moments in an increasingly difficult string of days, we did the right thing. He was ready, and I was finally ready to let him go.
I hope it's easy and a non-choice for you & that she just goes in her sleep. Still, if your old girl needs you to do that for her, your love for her will help you through it. Really. It'll be OK.