I have the same issue with our very old male cat who has stopped grooming himself. He is usually quite a mess - matted fur, poop matted on his bum, etc.
We are just very consciencious about it. We get him groomed, we clean the poop off his butt. We just do what we have to.
I would like to ask a hard question and I hope you aren't offended; but are you sort of looking for someone to say 'put her down' so you feel okay doing that?
Again, I apologize if I offended you, I just hear this alot (that's why we have eleven cats) and as I'm sure you know I don't like beating around the bush.