Becky, I have 2 mares who have Cushings, and several in my herd who are insulin resistant. All are managed and 'healthy' now, although the management is ongoing and requires periodic tweaking. Some of the symptoms overlap, but it is important to understand the differences. Check out this web site, which is a fabulous resource for all things Cushings/IR.
www.ecirhorse.com
It is based on more than 10 years of intensive research on thousands of horses, and overseen by respected veterinary and other professionals. I totally trust this source.
Good luck with your mare, and if you want to chat privately, I would be happy to.