I am so sorry for your loss. I know it is hard to think straight when you are off to the vet for that final visit
-- Your vet may not be as open to owners staying, or he knows how painful it is for so many people. He gave you time to say good-bye - hold onto that time. Please don't dwell on not being there in her last moment - rather hold onto all the wonderful moments you have shared with her in the last 15 years. I can tell you loved her very much, and I am sure she knew how much love you had for her.