When Is It Time?

Hello all. I adopted a senior Chihuahua this past July from an inner city shelter. I was told she had been abandoned but not much beyond that. I was told that she was healthy and house trained. I do have a history with adopting special needs dogs but this is my first senior. I got her home and quickly learned that she had apparently been severely abused and touching her tail or paws would cause a severe reaction. I also quickly learned that she was completely incontinent and she soon began wandering and getting lost and after a trip to the vet, I learned that she has moderate dementia, glaucoma and arthritis. We put her on two different medications and a supplement for the incontinence and neither has worked and I can’t use diapers because even approaching her tail puts her in an emotional state. I purchased an open air pen for the living room and one for the bedroom. I put pee pads down and she consistently soils them several times a day despite going out constantly (I’m retired from the Army and can stay home with her). She has become an incredibly picky eater and is skipping meals. I play with her every morning but after that, the entirety of her day is in bed. I can’t hold her in my lap because she’ll often pee on me without knowing it. Returning her to the shelter was never an option and she is very much loved but my mental health is suffering. My question is, how do I know when it’s time to say goodbye to a dog that I’ve only had for nine months? Thank you.