Years ago we had a little yorkie named Meg. She was only 7 but caught a really nasty virus and was very, very poorly.
She was at the vet's in their intensive care and we went to visit her every night. She was always pleased to see us, but so very sad. Every night we said goodbye to her not knowing if she would be there the next night.
On the Thursday she gave me a "look". I too cannot describe it, but in my heart of hearts I knew she was telling me that it was time. However, the vet had one new drug he wanted to try just in case - and I agreed, thinking it just might work.
All these years later I try not to dwell on it, but I still beat myself up about it because Meg died that night, alone at the vets.
He rang me in tears on the Friday morning to tell me she was gone and I felt desolate and so guilty that I hadn't been there with my little girl.
Having seen "the look" I will never make that mistake again, and if one of my dogs lets me know it is time, I will do what they wish.
However, in your case, it must be so very hard. I sort of believe the old adage, "better a day too soon than a day too late" as you would not want your dog to suffer......
But......
I really feel for you and hope you know when the time is right to do that last and possibly most important thing you can do for your old dog.