It is always very difficult to have a sick chicken, especially if you might lose them to it. Most of us on here are much older than you, and have kept chickens for many years, so we have lost somewhere and there. It is inevitable in keeping chickens. Sometimes we have to try to prevent them from suffering, and put them down ourselves. I always recommend that if you lose a bird (or put them down,) that you get a necropsy through your state vet, or at least do a home necropsy and take pictures of the major abdominal organs. We post those pictures here for opinions, to determine if anyone sees something that may explain what illness they may have had. No one likes to do a necropsy, but I have learned a lot from the ones I have done. I have seen one case of fatty liver disease, and cancer, water belly (ascites,) sour crop and possible gizzard blockage, salpingitis, and other things. There are good videos online which help you to identify the organs, and see what is normal.
