Can you try it from a different angle? Next time, perhaps you can be specific: "Mom, see that flattened squirrel 20 feet away? I want you to go stand right next to it and stay there until I tell you otherwise." If you see her approaching out of the corner of your eye before you're finished getting your horse where he needs to be, tell her, "I'm not ready yet! Please go back to the roadkill!" (obviously I hope you'll choose a sign or something, not roadkill, as your landmark of choice for your mom to hang out at)
If that isn't working, is there any way her horse is capable of going first over some of these obstacles, and then having her wait on the other side?
I understand how frustrating moms can be. I get frustrated often with mine. But at the end of the day, she's my mom, and there's no point in wrecking a mother-daughter relationship over the small bumps in the road, literally and figuratively.