Sorry to hear...
I can imagine your frustration. Our cat peed in a corner of our office, and we tried everything. It was a long time ago, so bet some of the products mentioned ahead of this are available now, which weren't then. Eventually it left, after repeated treatings with carpet cleaner.
Maybe your vet would have some suggestions, but bet the ones mentioned would be great. I do know that cat pee can be very hard to get out. I think it is Ammonia which their urine produces that makes it so; not sure on that, but pretty sure!!
Merry Christmas!
Mariah.