my horse has the same problem. I condition his feet with hooflex ever day and he also gets a daily biotin supplement. the hooflex has helped him keep shoes on considerably. However, it will take 6-8 months to start seeing results when using supplements. But well worth it in the long run!
My horse had a very bad cracked hoof at one point in time and I bought him HoofFlex and it worked amazing!
My thoroughbred had that problem and I started really soaking his feet with corn oil and it has helped considerably
Our ferrier has one of the horses in our barn using keratex hoof hardener for his hooves. It seems to be helping him. Maybe it could help your mare.
My farrier had me use icthamil. It's coal tar
I have the same issue with my mare and rainmaker seems to helping the best. Also she gets ground flax seed in her feed with helps with her feet and coat.