Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hi Keli,
Two things come to mind as I read your messages... First, are you overdoing it on the selenium supplementation? Selenium is highly toxic when given at too high a level. You must not exceed 5 mg per day. It sounds like you are giving her this mineral from at least 2 sources, if not more and there may already be enough in her hay (depending on what part of the country you are from).
The second thing is the high starch and sugar content of her diet. She is growing and this is not a good idea -- too much starch can lead to osteopathic disorders. Go with a low starch feed that has at least 5% fat. Then add flaxseed meal. If you are not feeding enough alfalfa, you'll want to buy a flaxseed meal product that has the correct calcium to phosphorus level, such as Nutra Flax. Flaxseeds that are not fortified in this way have an inverted calcium to phosphorus ratio, and you want to keep these two minerals in balance.
I don't know what's in the mineral supplement you're offering but you are providing minerals from several sources -- the "custom" mix (not sure what this is), the commercial feed, and the supplement for cushings horses.
You would be much better off feeding her all the grass hay she wants, along with 5 to 8 lbs of alfalfa, flaxseed meal, and a good vitamin/mineral supplement designed for growth such as Mare/Foal IIP. You can use a low starch commercial ration in a small quantity to act as a "carrier" for her supplements, or shredded beet pulp also works very nicely.
Her water consumption needs to be at least 8 gallons per day. If you are in a cold climate, it needs to be heated to at least 50 degrees. Most horses will not consume ice cold water in adequate quantity.
I hope this is helpful.
All the best,
Dr. Getty 
Where are you from? Bayfield, CO