When to Draw the Line - Feline Lung Digit Syndrome

My 14 year old cat was diagnosed on Friday with feline lung digit syndrome. 3 paws and 4 toes are already affected, swollen and painful. She is on antibiotics, pain meds, getting antiseptic foot baths twice daily, and wearing a cone. She is still eating, but not as eagerly as before and she is not asking for pets. Yesterday she didn't even lift her head when I approached, and this is a cat that would normally follow me around and scream at me, and then aggressively ram her head into my hand when I gave her attention.

She's not herself, that much is certain.

The vet said that the mean survival from diagnosis is 58 days. She had these issues for about 15 days, before X-rays showed the lung tumors and gave us answers. The vet said we could continue the current treatment, which is mostly controlling pain and treating secondary symptoms like infection. But it's metastasized cancer. It's not going to get better. I'm feeling guilt making her spend what might be her last days getting medicine shot down her throat and hiding, unwilling to walk anymore because it's still uncomfortable. But I don't want to make the decision too early, either, even though I know logically that even if her feet did get better than they are right now with the infection gone, she will just continue to get worse. The vet said to take it "day by day" but it's just so hard to not know the right thing to do.

How do you know when it's time to draw the line after your pet gets a terminal diagnosis?