Hi, as for work, the best is lots of low impact, low stress exercise. So motivating more exercise in the paddock is best if possible, and however many hrs daily you can manage at a walk & some trot.
If she is overweight, you do want her to lose some, not just not put on more, because its long term 'good condition' that is the major cause of metabolic probs, not necessarily obesity. While its not healthy for a animal like a horse to go hungry, free choice hay that she can gorge on is probably too much for her. Feeding about 2% bet daily of her *ideal* weight should allow her to get there & you can reduce that to no less than 1.5% if its still too much for her, or soak the hay.
Putting it into a 'slow feeder' net or such will prevent gorging & make it last so she doesn't go hungry. Also if she stands n eats all day, spreading the hay far n wide around the paddock in small amounts will help.
Oh & by the way, the saddle looks too far forward over her shoulders in that pic.
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