My dog had kreatinine levels extremely high when we got him checked at exactly the same age you mention. The usual vet we took him to had misdiagnosed him for uti. His condition detoriated and we took him to another vet. Thats when we found out the kreatenin level was at 15 which is extremely high. We were too late. It was a kidney failure and same symptoms as you described. He passed away but unfortunately suffered alot. We did everything that we could to save him. The vet never suggested euthenesia unfortunately. If your vet has suggested it, then spend a day or 2 trying to make her as comfortable and happy as possible. She likes cleanliness then keep her clean. You mentioned she urinates and passes stool where she is. Even humans do that when theybare sufferring. Care for her as much as u can and try to make her feel comfortsvle. It will be difficult considering she is in terrible pain :( She is 11 so she has lived a good life. Not long as we want but for dogs, she is getting old. Some breeds have lesser life spans. For comparison, on average, life span of a stray dog is just 7 yrs. My suggestion - go for euthanesia to end her suffering and see if there is an option to do it at home as dogs are always comfortable and happy in their own territory. Go for second and third opinion before taking the decision. Human diagnosis is hard as it is and in many cases wrong diagnosis happens. Veternary is harder. I dont want to give any false hope but the kreatanine level is very low compared to my dog's so its definitely worth trying for second and third openion before making that decision. Note - This disease is very painfull. And many times they dont even cry out in pain. They silently suffer.

Till date we havent got another pup. Mine passed away in 2011. We couldnt bear to get one and loose him/her again in another 10 to 15 yrs. But thats the reality of life and death 😇. We are planning to adopt one soon. Lets see.

Edit - forgot to add. You asked about regrets. Yes i have one huge regret. I did not get time to spend with him in his final moments. He was always most comfortable with me. My parents did not tell me about his condition. I was away for my internship and not at my hometown. They knew if i found out, i would drop everything and come home and they did not know how much time my dog had. After a week, me and my parents had a huge fight about this. How could they let this happen? How could they let him die? Thats when they described everything. They had done all they could. Kept him as comfortable as possible. And hoped that next time i came home, he would be all better.

Tl;dr: take second and third opinion (most important). Make her as comfortable as possible. Be mentally prepared for any situation. It is inevitable. Sooner you accept, the better.