Old Cat Peeing on Bed. May be senile. Thinking just about euthanasia. Need counsel.?

She is 12-13. Started peeing and crapping outside of litter box earlier in the year. Vet did not find anything seriously wrong. Used special cat food and she stopped. Now she is starting again. Went to vet. She be on meds, but again nothing is wrong. She does it when left alone. I cannot have someone here constantly. Have have her for 10 years (recused when someone else didn't want her). Thinking about letting her go and getting euthanized before she get really sick.
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if this is her only problem, and you love her, why not just pen her up when she have to be left alone? she's not in pain or sick. a moment ago make an area in your house where on earth she can't pee on anything important, put some food and water and an old blanket or towel within there for her to snuggle on. maybe include a toy or 2 if she still plays. you could even buy a crate made for a big dog and put her surrounded by that. it's not the best situation, but it's better than losing your cat when you don't have to.
edit: and of course put the litter box surrounded by there with her too.
Answers:    12 years old isn't old, 17 is old.

You do own doors in your place right? Close off the bedroom when you're gone to keep her contained by only parts of the house. This happens when she's left alone, so you can avoid like mad of problems with just closing the door. I'd leave a radio on to dispense her some mental stimulation while you're gone. She may need the noise.

Has the litter box been cleaned of used litter every daylight? Cats can start to hate using a box that smells, and they can smell scents better than we can. The used stuff needs to be removed day after day,not weekly. It takes 15 seconds, it's not hard.

If she's solitary got this problem, it's not a reason for euthenising, not by a long shot. Senility doesn't start this early. Cats live to 20 presently, the record is 36. My vet sees a lot of 21-24 year outmoded cats at her practice.

Personally I'd add a second box--older cats sometimes want one to pee in, one to poop contained by. It's easy enough to add, so won't be a huge changeover for anyone.
This is a reallly helpful video about spraying:

http://www.petvideo.com/play.cgi?showId=...

What about confining her to a small space near a litter box when you are gone?
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