I am so very sorry, too. I've been where you are way too many times already and will be there again. Just lost one the other day to it, though she seemed to be putting her weight back. Sometimes, they'll go up and down and you grab hold of some hope, then they just go. It's always heartbreaking, but you get to know the signs.
Thank you, everyone. Knowing the signs now, hopefully, if this happens again, I can combat it a little more/better earlier on. I hate that I had to learn this lesson with one of my little birds, though.