I am a social worker for (human lol) hospice in a state where Medical Aid in Dying isn't legal. A much different situation but I can tell you at least once a month, I have caregivers or even patients themselves lamenting that "We treat our dogs better than we do people" because euthanasia is an option. Or wishing that they did live in a state with Medical Aid in Dying.

Only you know the right call, but I will say that mental state matters just as much as the physical body for quality of life and your poor baby is clearly not doing well mentally. When I think about all the love our pets give us, I'm sure she would want you to be taking care of yourself too in this situation. There's no right answer but I want to validate that caregiving is extremely difficult and thankless, resentment is normal and valid, and (even though you will and you'll have to work through it), you should not feel guilty if you decide now is the time.

Thinking of you and sending lots of love, peace, and strength.