The lethargy could be related to malnutrition and she just hasn’t had the dramatic weight loss. Yet. Can you get her hot water? Even if you have to bring an immersion heater and warm it yourself. Lot of horses won’t drink icy water and dehydration sets off a whole cascade of problems. And by hot I mean over 50f, not hot hot.
But don’t mistake this for a majikal, easy solution. It’s not. This mare needs to see a vet. Soon. They don’t bounce back like a young horse and once their health starts to hit the skids, it often keeps sliding. I wouldn’t fool with waiting, you need to solve this ASAP.