I used to ride a trail horse that would often yank the reins out of children's hands, because he decided it was time to stop and graze. I then started riding him with a dressage whip so that every time he tried to stop and eat I gave him a good whack on his rump. He stopped doing that after a couple times and now he is a lot more respectful. The only problem with this is that each horse is different and if your mare is already nervous, then this may not be the best method. You do not have to hit her hard though, just enough to get her attention.