When you practice your leading skills at home, does she nap at you then, too? Are you using a chain on her nose.? I know that makes Mac crabby.
But, ulitimately, you will have to work with her more on this and being careful to set boundaries and reinforce them , that says to your mare, "your head doesn't pass this imaginary line." If it does, then you tap firmly on her cheek or nose and "bob" her over . It doesn't hurt but is annoying and a sharp reminder that she just crossed the line. But, you must be consistent. If you do, then just your raising your fingers, and using a scolding voice to say, "no!" or whatever you choose (I say " ah ah ah!"), will be enough to remind your mare to stay in her space, not yours.