I think we all understand how you feel. Please don't beat yourself up too much. Animals, unlike people, aren't able to vocalize what they need or how they feel that well. Signs of illness or injury can be very subtle and even more so in birds as they often "pretend" to be well - ie feign interest in eating, drinking etc - either to protect themselves from being picked on by healthier peers or just as a ingrained behavior. Regardless, they can appear well and not actually be well resulting in a delay in diagnosis and treatment. I know it is heartbreaking to lose a pet. It's easier said than done, but try not to feel guilty. I lost some hens to a pair of red tailed hawks recently and felt so guilty about the fact that I had noticed one of my pullets ducking more and more alert to what was above her a few days prior and I had not recognized this as a threat from above until after they had been killed. You have provided a good home for your chickens, tried to nurse your hen back to health when others (and this isn't wrong either) would have culled her, and truly care for her well being. Most of us are not chicken experts and so we unfortunately learn through trial and error. It happens. And sometimes, regardless what we do, we have an animal who is more susceptible and sensitive to illness that cannot be saved. The most important thing for a bird is while treating them, hand feed them or closely monitor food intake as they do not have the same reserve that other animals have. However, if she is starting to have labored, shallow breaths, I think her time is soon (if not already). If this is the case and she is beyond hope, forcing her to eat/drink just causes more discomfort. Instead keep her comfortable and wait (if you don't think you can euthanize her). We feel your heartbreak.