...why are you riding on gravel? Who rides on gravel?
Very few horses can march across gravel without feeling uncomfortable. The sharp and uneven surface of the rocks digs at the sole and can pit it, plus it can bruise the sole. Even a shod horse will bruise if it steps on a big enough piece of gravel.
Avoid gravel for riding...
Anywho, have your farrier stop taking off so much sole, slap Venice Turpentine on after a trim and once a week if you need it. If her hooves continue to be sore, spray with iodine daily to help toughen them.
The boots available for riding are: Easyboots, Boas, Old Macs, Cavellos, and Renegades. Any other boots are not widely used and may or may not be useful to you.
I'd suggest Easyboot Epics for you. They're fine to ride in. Most horses only need the fronts booted anyway. If you need the backs booted, I'd suggest Easyboot Gloves. They're cheap and there aren't any metal parts.
If you need something for turnout, go with Easyboot Gloves. Make sure you fit them properly.
Stay away from Boas. They cause bruising to the front of the hoof wall for most people.
Cavellos and Old Macs are fine to ride in.