I am really sorry to hear about this. Last year at around this time, we put my Yorkshire Terrier of 18 years down, and it was very difficult, since I had had him for so long it was actually impossible for me to remember a day without him.
We put him down because he became senile. Listen guys, euthanasia is not a bad thing. It really isn't. My pup was sitting in his water dish, couldn't see or hear anything. Life was nothing to him anymore.
When you adopt a dog, you are essentially becoming their guardian. You have responsibilities not just to keeping them alive as long as possible, but to ensure the quality of their life. When people get old, many times they are put on life support, and it is up to their guardian to decide what to do. It's actually more like torture to keep a dog going, when life isn't fun for them anymore. Like you said, they can't voice that they are ready to go.
I am very sorry to hear of your four legged friend, but you did her a great favor, and she is much happier now! All she ever wanted for you to do is be happy, and even in her passing I'm sure she would want you to focus on the good times.