I’m sorry that your best friend died. My 17,6 years old dog had pts 15 monts ago. People think that easier if an old dog died.
Maybe if they oversleep after happy life without have to pts but I don’t know. The euthanasia makes the things more complicated.
My Sophie also was old and deteriorated but as same as you I also feel that I failed in some way. Don’t have clear grief because I have lots of guilts.
I know I could have done some things better and this make me very depressed. I know what you feel.
I feel sorry for your loss. Liza