Sorry Tom to hear about having to put down your dog. It’s not only a tough decision of when you should do it, but also how you should do it.
Obviously you care a lot about your dog otherwise you wouldn’t have taken the time to post this question and you certainly wouldn’t have cared for her for all these years.
No man should judge another on how we deal with death. I can certainly understand that since it’s your dog, why would you want someone else to put her to sleep? After building so many cherished memories and for being together for all these years, I can understand how you may feel that you owe it to her.
Whatever you choose, I hope you find some closure and peace of mind that you did the right thing for her and for you.