I'm so sorry for your loss. It's incredibly painful to lose a companion, let alone one who has been such a huge and long term part of your life. 15 years is a long life for any dog and shows the unwavering love and commitment you have had. Take a moment to remind yourself of all the love and effort you put in over all those years, because not everyone does, and it is commendable.
I lost my best friend, my dog of 13 years, just over 4 weeks ago now. And like you the very first thing I felt was intense guilt. I also had to make that impossible choice to say goodbye. And, like you, I spent the last year of his life nursing 24/7. We had many hospital stays and multiple chronic illness diagnosis over that last year and all those illnesses compounded on each other making things worse for his health. The week before we said our last goodbye, he had come home from a 2 day emerg hospital stay and had another diagnosis (diabetes). When he came home I actually had hope, this was treatable/manageable and a few needles was nothing compared to the nursing protocol I religiously followed over the previous months. However, within 1 short week of being home he started coughing up blood and clots. When his specialist said he assumed cancer and I was faced with having my boy go back into the hospital (during this crazy covid situation when you cannot be there) and undergo more invasive testing to confirm - I looked at him and said "enough". I still can't tell you why/how I went from a year of heroics to some gut instinct taking over me to protect him from further suffering, but it did and I made the call to let him go peacefully a day later.
I recently said to a friend that owning a dog is the most responsibility we can take on; we love them, raise them, nurse them through age and then play god. I think anyone who can do that without second thoughts and emotional guilt, prob shouldn't own an animal. The truth is we can never know what the future holds, not for us and not for our dogs. But what I try to remind myself is that for 13 years every decision I made for my boy was with pure love and in his best interest. You should do the same. You loved that dog with all your heart and you let him go in peace.
My best to you and your family. xx