I am so sorry for your loss. I have recently been through the same thing with both of my dogs. In less than 3 months I had to make the heartbreaking decision twice with the second one being only 2 weeks ago. They were 16 & 17yrs old and both suffering arthritis making it very difficult for them to get around. I am still second guessing my decision. I am still grieving for them and feel guilty.
From your post, it sounds to me like you made the right decision but I understand doubting yourself because I have been too. remember what a great life she had and how loved she was.