This is a hard thing for anyone that loves their animal. Just to help you with your decision and trust me my animals will live out their life as long as they can I would take her to the vet to make sure she doesn't have a kidney/bladder problem or just get a wellness checkup. I would then invest in these panties and put light day woman's pads in them.
http://www.joybies.com/
I think these are so much more comfortable for a dog then the diapers and they are cute on. I would really hate for you to get frustrated with her on something she can no longer control. I know you would feel awful.
I am a big believer, like you, as long as the animal is not in pain they will live. They have given us to much just to put down because they are going blind and are losing control. I can only hope that they die peacefully in their sleep.
My thoughts will be with you.