I can understand how you feel. My RB Sasha was 15 years old when I nocticed she just wasn't herself and took her in and the vet found all sorts of problems with her. The decision to have them PTS is the hardest thing to do, because you had her so long, she's like one of your children. I just knew in my heart that it was her time, so I took her home and loved her and let the kids spend some time with her then took her the next day to be PTS. I made the appt. at the end of the day so that my husband could go with me and we could go bury her out in the country where we plan to move. That was tough and I cried for days. A couple of weeks later, I saw a shadow out the corner of my eye coming into my room and it was her. I even felt her get on my pillow above my head one night, that is where she used to sleep. I still miss her a lot and that's been 6years ago, but she was like one of my kids.
I think you will know in your heart when it's time and she will be ready. Just keep doing what you're doing as far as letting her have what ever she wants and give her lots of love. God Bless you and Dusty.