Good advice from all. Poultry vitamins containing riboflavin (B2) would be best, and that rules out Poultry NutriDrench. Mareks could be possible at her age, vitamin deficiency, or a possible infection or egg binding are all things to consider. Keeping her in a crate close to food and water, then letting her out often to judge how she is walking are good. I think you are dojng a great job, and it is a big deal treating a sick chicken by yourself. Feeding wet grower feed, cooked egg, tuna, or liver in small amounts are good. Keep us posted on how she is doing. Here is some reading about Mareks, but hopefully, that is not the issue:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/