Oh, Pezzy... I'm so very sorry for your loss. You tried so hard to save your little friend. Please try to take comfor in that, and in the fact that she had someone that truly tried to take care of her and give her a good life.
I'm so sorry, Pezzy. If you have the stomach for it you might consider doing an autopsy to determine the exact cause. Sometimes this is the last thing our goldfish can do for us, teach us what went wrong so that we can learn from it.
Best of luck to you in the future.