It's time...or is it?

After 17 years, I believe it's time to put my sweet kitty to sleep. For the last 2 months or so, she has been forgetting where the litter box is. She will wander around and meow until I pick her up and take her to it. Last week, she started sneezing blood. I already have to give her pills twice a day for her thyroid, so it just seems like her quality of life is declining. I'm so sad about it, and just very torn about whether this is the right decision. With people, this is not an issue, so I am left wondering why I am given the power to make this decision with my pet. Shouldn't she just go naturally as well? Am I assuming a power that God should have? I'm just so torn and sad.......