I have talked to so many families who agonize over the last several days with their pet. Deciding when AND if to end a pet's life involves a good understanding between you and your veterinarian of the likely progression of the disease your pet is struggling against.
You need to weigh a number of factors:
-your pet's ability to eat and drink
-their current quality of life
-your ability to control their pain/discomfort
-the affect their care has on your life and finances
Making yourself ill from lack of sleep, putting your job in jeopardy from an inability to work, accumulating debt you cannot reasonably repay or hurting yourself from lifting/carrying are all valid considerations.
Your pet is your love and your companion...ask yourself, would they want to hurt you, ever?