Over the winter the herd dynamics changed. You are trying to force her into a position she is not longer comfortable with. It seems the other mare has become more dominant than your mare. You can try and work her thro it or ride a horse that's much happier to follow.
this seems likely, but in my oppinion she should veiw you as leader, and accept your direction. I ride all members of a large herd, from the alpha mare to the most submissive underling, and they all, without exception, can ride out alone, in the front, middle or back on the trail.
She doesnt trust you, and beleives you are putting her in danger would be my guess. You need to go back to your CA groundwork, and start riding only when she is fantastic on the ground. Take baby steps under saddle, working her at first in a small area, where she feels safe. Work up gradually to further and further away.