Failing is part of a learning process. Think about other new things you've tried and how many times you failed at it before you got it right. That's just part of being human and it happens to us all, every day and in many ways. This world tends to frown upon failure or those who fail like it's a bad thing or that they are bad people, but I tend to see things from the other end...courage is trying it in the first place. Intelligence is knowing you had a problem and couldn't handle it in the way you had wanted. Compassion, courage and intelligence combined is what makes you want to do it better next time and are not afraid to talk about your feelings on it all.
I approach my flock tending from the other end of the perspective as well and I'll tell you about it, as it may help you avoid dealing with seeing a hen go through a bad death and feeling helpless about it.
I cull on a yearly basis and use the criteria of good laying(must lay every day or every other day in peak laying seasons), healthy appearance and vigor, feed thriftiness(doesn't overeat...a fat hen is not necessarily a healthy hen and it can lead to problems with laying and overall health, just like in us humans), and temperament(is she a continual bully? Usually these are not your best layers because they have a little too much testosterone. Is she picked on by all the flock? They usually know before we do if there is a hen that carries heavy parasite loads or hidden illness/deformity).
This yearly culling is done on hens that are not sick yet but have the potential of becoming ill later due to old age, hidden problems or genetic issues, and/or parasite loads(those that carry heavy parasite loads are usually lacking in good health in the first place, making them more vulnerable to infestation). This way the hen being dispatched is still feeling well, can be safely consumed and the death is controlled and managed well so as to provide the least amount of suffering.
This leaves one with a flock that holds very little surprises or drama concerning health and death, so no drain on emotions and stress. It also makes for a better flock life, with less infighting, pecking order issues and a better social structure.
I don't feed special treats or amped up foods for their consumption, as they fair better on a more natural diet and existence, much like the wild birds. The most healthy flocks I've ever seen were those that lived rough on the land without one ounce of pampering, so I try to restrain my own propensities to mother everything within a 10 mile radius.
I do try to get freshly milled feed, then I ferment it, but it's only a supplement to their natural diet they find on the land, so it is not offered free choice. Fresh soils, a natural bird diet, exercise, fresh air and good social lives can help keep a flock healthy when all else fails. I've never had sickness in my flocks and I've been raising chickens for almost 40 yrs now.
All of these things I've told here is another way of approaching your flock's needs and I tend to refer to it as preventative vs. curative. Even vets have a hard time curing problems in poultry, so always leaning on curative measures sets one up for failure more often than not. Failure in one's efforts can be a positive thing when it makes them take stock of the situation and eager to make changes to prevent further problems.
Just things to think about and ponder for the future. It could be that you've never heard of other methods of raising chickens, so I offer this to you as a possible alternative to the current trend of waiting until birds get sick before trying to help them.