I had such a problem too.
What I did:
I just sat on my horse and whenever she wanted to go away, I pulled up and told her to stay. I did this a hundret times and suddenly she got it. She didn't move away, until I told her to.
I don't know if this helped, but you sure could try. (:
You could train this in the square or somewhere save.
Like for example put something uneven on the ground and walk her over it. (walk with her) Or first you and than her. (Maybe she'll follow you)
Does she have like a best friend or something?
Maybe than you could take this other horse and walk over with her/him. So probably your mare will go over there too.
"I don't whisper to horses.I listen to them."