How can I make my cat's final days comfortable?

My beloved 16 year old cat, Spoony, had a seizure several days ago. After that, she completely stopped eating. Friday evening, we took her to the state-of-the-art “MSPCA Angel Animal Medical Center in Boston. The place rivals Massachusetts Geranial Hospital in size and equipment. They even have MRI machines and staff neurosurgeons. Although the trip there in her weakened condition stressed her out something terrible, we thought we should do what we could to keep her going. We wanted to know if there might be a way to save her. They did a complete battery of tests. The doc called today, and the news is not good. She has acute kidney failure and hypothyroidism. The treatment of hypothyroidism puts added stress on the kidneys and makes that worse. To complicate things, she has a severe heart murmur.

The doc wants us to bring her in again so they can teach us to give her injections every other day to try to hydrate her and keep her alive longer. She’s terrified of the trip and would hate the injections. She is so tired she can hardly move and just wants to sleep. We don’t think anything we do will keep her going much longer, and it’s pretty clear to us the stress of holding her and injecting her would make her last days far more traumatic than her health problems are already making them.

Is there anyone who has faced this and learned a way to let your pet go as peacefully as possible? We don’t want to take her in to be euthanized, nor do we want to do it ourselves. Cats have nine lives, and she just might make a miraculous recovery. We want to be with her and make sure that if this is her end time, it is as peaceful and painless as it can be, not one handled by strangers who terrify her. Is there any analgesia I can legally obtain and give her to let her go through this painlessly?