<human>: Can you give me the gist of the text in a nutshell?

My cat is about 16 years old. I've had her since I was a young teen and she's been with me through school, 4 serious relationships, and a cross-country move. She's my baby girl.

But she's never been a healthy cat. She was a pet store impulse buy and never very healthy from day one, so though you might be thinking "16 isn't very old", she basically reached elderly critical mass several years ago. She weighs about 5lbs now and is practically falling apart. All her vet check ups come back fine, she's just old. 

In the past approximate 8 months she's stopped using the litter box. She'll often just go where she's laying, and if she does decide to get up she'll just go standing in the middle of the room, or sometimes make it to the nearest corner. 

Believe me, I've tried so many things to get her interested in the litter box again - changed the litter type, cleaned out the litter box much more frequently, bleach-scrubbed the box, bought a whole new different shaped litter box, moved it to a different area, tried to make the area quieter or more inviting somehow. My house is now filled with boxes and tin foil in "funnel" shapes that try to make it easy and obvious for her to find the litter box. The house is very small (700sq ft) and the cats are only allowed in 2 of the rooms, so it is no more than an 8ft walk from anywhere to the litter box.

The last thing I want to do is have to put my cat down because she's pooping and peeing inappropriately. Every living creature gets old and this stuff happens. But I don't know what to do anymore. I spend so much in effort/time and money cleaning my carpets and furniture, and when she dies in a few years I'm going to be left with no cat and filthy floors.

Aside from keeping her in an empty room (which I just do not have) or a dog crate/ferret cage, I just don't know what to do anymore.

Can I get some advice from someone else that has had to deal with this?
<bot>: Cat can no longer control her urinary or bowel movements, house is a mess, and I need help making a responsible and ethical decision about this.