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
Don't give it a hard feed only hay
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.
If you want your horse to lose weight try putting it somewhere were it has little feed If possible
Why do you want her to loose weight?