Don't know if it was already said, I didn't really care to read many replies.
But, I'd say that she's just pushing your buttons, unless there is some ulterior motive for her to not move forward such as tack. When my horse wants to back up incessantly, I pull her backwards, because then she's doing what I want. So then she goes forward when I tell her to. There isn't a need to fight your horse, so, instead of LETTING her go back when she wants, MAKE her go back when she wants.
If you wear a breast collar, that may make her back up, I know a horse who wouldn't go forward with a breast collar, but once it was off she was fine!
"Doing what you like is freedom, liking what you do is happiness."