Watching Her Fade

There's nothing quite like finding out your cat puked by stepping in it!
About a month ago I was petting my families beautiful persian, Prissy, when I felt a lump on her right hip about the size of a marble. We immediately set up a veterinary appointment to get it looked at. Our worst fear came to pass, as we were told she had cancer. The vet gave us some options that we could go home and think about. None of which seemed at all promising.
Within a week the lump had grown to the size of a golf ball! We took her back to the veterinary. The vet was shocked to see it had grown so fast and told us there was nothing that could be done at this point. Every day, I watch as Prissy becomes more lethargic and her hip becomes more tender, and I wonder how much longer before I'll need to take her back to the vet...for that last visit. I've never had to make that final decision for a pet before...How do you even begin to decide?
One day she'll be looking really worn out and sickly and the next she perks right up and wants to be petted by everybody. She's still eating and seems to mostly be enjoying life. On the other hand she is puking more frequently and pooping outside her litter box about every other day and her hip definitely has slowed her down. But just when we think her time is just around the corner, she'll do something totally adorable which makes it impossible to make that call to the vet's office. Yesterday, we came home and she was meowing at my daughter, Mariah. Mariah was ignoring her so Prissy put her front paws up on Mariah's legs and meowed at her again! Mariah was still just standing there so I commanded her, "Pet Prissy!" Prissy loved every minute of her petting and her favorite: under the chin scratches!
I stepped in some cat puke, but it was worth every minute of seeing Prissy's affection towards Mariah!