When is it time?

How do you decide when it's time to let your senior dog go? To preface, she has a vet appointment tomorrow to discuss everything.

My senior dog, Finley, is approximately 14-15 years old. she has Cushings Disease, Canine Cognitive Dysfunction, and is in congestive heart failure.

Currently, her heart failure seems to be fairly well managed, the vet says she has at least another year or so left with that before it becomes a major problem. Her cushings is harder to tell, however she recently had another ACTH stim test which showed that her levels were in the correct range with the medication.

She's on medication for the cushings, and is also on selegiline for the CCD, which has been getting a lot worse in the past few months. She's also been getting trazodone at night (which I recently learned was not safe to be given with Selegiline, I will be bringing that up tomorrow)

She's gone completely blind, and it's irreversible.

None of the medication seem to be making a huge improvement, despite being on them for several months and adjusting dosages (Selegiline less long, however she has had no improvement on it). She is constantly peeing and pooping everywhere, almost hourly. At LEAST every 2-3 hours. She gives no signal that shes about to go, either. She doesn't even try to go on the pee pads she has. She just stops, squats, and goes, then walks through it.

She has a really hard time sleeping at night. She'll often lay in her crate and just cry/howl. She'll get up and pace, seemingly unable to settle. Sometimes she'll be doing something, and will randomly pause, being completely still, then she'll resume her activity. She's recently started walking in circles at night, too. With her being so confused at night, she often doesn't want to sit on the couch with you. She'll try to fall off, or she'll just whine. If you hold her still long enough, she will eventually lay down, (not always), however, she sometimes just lays there with her eyes open and whines. She can't just wander the house or even just a free room because she has such a hard time navigating, and will also try to eat anything on the floor, even non food items. She's even chewed on chairs before. She mostly stays in a large pen in the kitchen with her pee pads, water, and her crate with soft blankets/beds.

As I'm typing this, I'm really not sure what the appropriate next steps are. We've been chasing down symptoms with her vets for over a year now, and nothing seems to help her. We've spent so much money on tests and medication. She's unhappy so much of the time.

But then there's times where it's like I can see glimpses of her peaking through all the fog. She'll get excited and play with a toy, she'll fall asleep on the couch with you, she even will respond to commands for treats (this one is less often, but still does happen).

I don't know what to do. I don't know at what point it's more fair to let her go instead of her being so confused. I don't know if there's any realistic way to improve things for her past what we've already done. I really don't know.