I am always a firm believer in you will know when it is time. Have you spoken to her vet on his opinion? I am so sorry you have to make this decision it is so very hard to do.
I agree with this...speak to the vet.
I will tell you about my last situation, concerning our beloved Katie.
Katie was a GSD rescue we saved and loved dearly. She was one of the most loving dogs we've ever had. Unfortunately, she developed most of the problems you have described here. I personally could not bear to see her suffer any longer...my personal beliefs of quality over quantity of life are clear for me.
After many vet visits and meds, I knew when it was time.
My hubby, on the other hand , couldn't let her go. Hubby took her to the vet and came back home with her and even more meds. I called the vet and he stated that hubby wasn't ready to let her go so the vet gave her more meds...I think he did that more for hubby than Katie
It was agonizing for me to watch her suffer but, I gave it one more month.
Her breathing became so labored from the meds that I ended up taking her to the vet by myself for her last time. (Hubby could not bear to go and said his goodbyes here at home)
I sat on the floor with her and held her until she fell asleep. I think she knew and she was ready...
I had her cremated and her ashes now sit on our bookshelf with our other beloved GSDs...we both still come to tears when we think of them all.
I Know the heartache you must be going through and I'm so sorry for your pain...no one here can tell you what to do.