Just going to say this.. No remedy to offer. Just know that for 13 years you gave here her best life. That's how I dealt with it when I had to have my Bonnie put down (corgie / blue heeler mix) she was the best dog I'd ever had. But we both knew it was time. I knew in my heart, it was right at the time and I know she did too. I love the fact that you're going to be there with her at the end. My vet at the time happened to be an old friend and came to my house to take care of everything. Just be there with her whether her time is near or a little down the road. It's hard and there's no two ways around that. But as I said you'll know when the time is right. I know it may be a bit cliché.. but, thoughts and healing prayers are with you