Soak your hay as it takes all the sugars out of it. I have a horse called Chanel who has Cushings and ems so she has to have no sugar and that’s what i do to get the sugar out of her hay. It really works. My horse Chanel hasn’t lost weight in 4 years and in the last 1-2 months she has lost quite a lot because I have been soaking my hay
If she is living out and unclipped then dont rug her (if she is clipped then put a lighter run on if its colder) and dont give a hard feed, lots of trot work for longer periods of time and if she is living in then a double wrapped and soaked haynet :) hope that helps in anyway
Maybe check what you're feeding her and talk to your local feed store for a weight loss supplement or a feed with less energy. Exercise her more often and for longer and maybe if she's stabled put her in a paddock with just a small amount of grass with other horses so she can have a gallop on her own and burn some energy.