I asked our Vet that same question, and he told me the same, "she'll let you know"...... She was diagnosed with Cancer at the age of 10, lived 5 more years.... When she started having trouble with her walks and having accidents plus she had quit eating for the most part.... She gave us that look that said "I've had enough"....... One of the hardest things we've ever done, but it is the most humane thing for them..... The Vet came out to the car and gave her the shot and she went to sleep in my arms...... That would have been Shelbea, but with Abigail, she got sick and went down hill within weeks, we waited a couple of days too long, she ended up having a stroke and seizure then dying at home... Was one of the most horrible things to witness.... My heart broke... She suffered.... We were going to take her to the Vet the next day to have her "go to sleep" . If you think she's miserable then she probably is.... Good luck with your decision...... Thoughts are with you....