This post is spot on!As a newbie, and a general control freak, I am coming to understand that it may be that to a certain extent these things with chickens are beyond our control, and even more so than with some other animals, because chickens are a prey animal and they are so good at hiding their ailments, until they are right at death's door. But I also see that many things are not controllable, even when we know what is going on ahead of time. A lot of these ailments are due to systemic failures that are not curable. We cannot fix it, and more importantly, no one can fix it.
Sometimes, the most anyone can do, and the most anyone - including yourself - can and should expect from you, is to give them love and comfort when it is clear there is no more to be done. You performed that care very well.
I may understand your feeling of shock and devastation, having cared for someone's health myself and not been able to change the course of disease (in both humans and pets). It is very hard to accept this devastation, and there's a part of our brains that wants and needs to find a reason for why we feel so bad, and it is easy to get into a coulda-shoulda world of blame. Don't go there. It took me awhile to understand that that can be part of the nature of grief. It takes time, and living through it, to deal with it. You need to grieve for her and yourself, for your loss of your companion. Try to remember to be kind to yourself in this process.