I somewhat agree with the above poster. Whenever she rears make it dramatic and do almost anything to get her to go forward.
I'm not a big fan of backing a horse up when they aren't listening. It puts strain on their legs and it can be very dangerous for you. If the horse decides all of a sudden that they want to spring forward or rear, you are directly in their way and you are going to get hurt. Another thing, my horse was trained with this method in mind, but every time she resisted they would back her up. She's got quite the hind end on her and a compact body so that's much easier for her than pushing up into the contact. So now, when she's pressured, she'll go flying backwards and when I'm on her, there's nothing I can do to get her to stop apart from steer her into a wall. She's figured out that if she doesn't want to do something she can back up and I can't do anything about it. Last time, she actually backed out of the arena and started panicking when she started sliding on the concrete. The only thing I could do was tell people to watch out, calm her down and walk her back in without getting off. I think she scared herself though, because she hasn't tried anything remotely close since.
I hope all goes well with your mare.
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie