Your need to learn to hold the foot securely in your legs just above your knees so you can use both hands on the rasp. this takes a lot of practice to keep the foot steady and in a position under the horse so he is comfortable(not pulled out to the side) It requires a lot of bending and twisting of your knees at the same time. Then the hand on the front end of the rasp (the non handle end) should actually be the one guiding the rasp and (most important) controlling the pressure. The sharper the rap, the LESS pressure you want to use. So with a new, sharp rasp VERY LITTLE pressure is needed to rasp off a lot of material in a short time. If you push too hard it only digs the teeth in too much and the rasp can't move forward easily and just jams up.
The next important thing: the rasp ONLY WORKS going forward, NOT EVER when moving backward toward the handle. If you have pressure on it as you pull it back toward the handle you immediately dull it and it gets no work done that direction anyway. So on the back stroke let it "float" with no pressure at all. Just like a saw, if you have ever used a hand saw.
Also when you rasp the heels, to prevent the foot from "chattering" due to the rasp catching on the edges of the heel, turn the rasp around backward and PULL it toward you, from the non handle end. That part you can do one handed.
Hope this helps.