Miss Mary, now you have a choice. Do you let her linger and suffer until she dies or do you give her the hand of mercy? If you love her as much as you say you do, then mercy is the loving thing to do. No, it's never easy but love of that kind never is.
Just because she's drinking and walking around, it doesn't mean she is getting better or isn't suffering, it just means she's drinking and walking around. I usually give a chicken a few days to see if they will snap out of it, but after that it's just not likely to happen.
Don't mean to sound uncaring about how you are feeling...I've been there many a time and have had to kill quite a few of my favorite hens and roosters~just because I regularly eat my chickens doesn't mean I don't have favorites I consider more of a pet than the rest of the flock. Many a time I've had to kill with tears in my eyes and an aching heart. But, how we feel shouldn't take priority over how the animal is feeling during these times.