My heart goes out to you. I totally understand.
Our dear late Sheba died at 24 of kidney failure. I am still guilty about it ( Oct 2010) because I think I should have put her to sleep earlier. She never cried with it though.
Cats that vocalize alot I'd worry about. It can be a sign of hyperthyroid disease, which strikes elderly cats as well.
Last Friday our 14 yr old Oreo's cancer leaped out of remission and we had to put her to sleep too... this was very traumatic.
I cried all day long on Friday, in fact...
It is never easy to release a beloved pet from pain. But in the end the decision has to be made. If you wait and let the cat suffer, then you will suffer GUILT...If you do it now, you will wonder if it was too SOON. I don't think there is a clear answer.
But if your kitty has a very high creatinine test result, I think your answer is to release her. That is just my opinion.
My profile album has photos in all 3 categories of our pets, if you would like to see them.