I'm so sorry for your loss of your wonderful companion. As owners, we are often put in the position of choosing when our beloved companion's life will end. You mentioned that you will always second guess your decision, please don't! I read your post and thought how wonderful Cocoa's life sounded, and wished ALL dogs could have such a joyous journey with people.
You gave your dog an amazing and wonderful long life, and although the passing was hard, you gave her the dignity to end her life before it became unbearable.
My prayers go out to you in this very hard time of your life.