She may just need a better mustang roll around the edges with a touch up between trimming. Your farrier should be able to show you how to make a roll around the edge of the foot. It's not that difficult.
If she's getting sore, then she may need protection. Personally, I prefer boots over shoes. Boots allow you to protect her feet only when she really needs it. You don't have to worry about a lost shoe, added risk of thrush/WLD, and it's cheaper. A good set of boots will last a long time, paying for themselves quite quickly. Plus you can still ride her barefoot some, to help toughen up her feet, possibly allowing her to ride without boots in time.
Some conditioner for the outside of the feet may help with superficial cracking. I have had good luck with both Hoof Heal and Carona Hoof Ointment. Both seal in moisture while still allowing the hoof to breath, and they kill bacteria, helping prevent thrush.
A good hoof supplement will help her feet grow in stronger. I HIGHLY recommend Source Focus HF. I have seen fantastic results with it and my horses' feet grew out VERY quickly. I used it when my mare had some wall separation and it really did wonders.