I'm not a feeding expert, but I would personally suggest considering putting her on a feed for weight control. I'm not sure if it would differ because she is a mini, but I have a full-size horse who needs to lose weight and I've found a lot of good out of switching her into Purina Well Solve Low Starch grain. Honestly, you don't want to harm your horse by restricting too much. I know your horse is small, but she needs to still have a nutritional diet. I would recommend contacting a vet.
I am curious; while Wellsolve is certainly much better than high carb/high fat/high sugar feeds, why did you opt for the Wellsolve instead of something lower in calories, such as a vitamin/mineral supplement or a ration balancer?