I've seen it the other way round. My Keli had cancer of the nose and it had spread. Keli slept alot, had a few walks and ate. He was also on chemo.
The vet told me when she would no longer give chemo. I sort of knew. Keli did not want to go. He tried escaping and getting back in the car and refusing to go to the vets.
The guilt destroys me every day.
However, my OH's dog, Berti, was PTS in May. Bert also had cancer and on the last day his back end collapsed. The vets were sure it was not the cancer, but he was messing himself in his final hours.
I now know my darling Keli never went through that indignity and the cancer didn't kill him, painfully.
When you care very deeply for an animal, you put everything else aside for their care. It made me very ill caring for Keli with the stress it caused. Can the other people in your life still offer that level of care or are they also feeling the strain?
I didn't want to give up my Keli for anything, but after it was over, amidst the grief, I realised he was a sick old man. To keep him going was for my selfish reasoning, and not for him.
Only you can look at the situation lateraly and see if deep inside you are keeping your darling alive for you or for her.
This post is not what you wanted to hear, I know. You will know, but maybe you need to ask the right questions to find your answer, and only you know those questions.