If I am riding her alone then I do things differently, I ride with a loose rein until she picks up her pace from a walk. Then ill correct her and put her back in the walk and continue riding with a loose rein. But eventually she gets sick of this and starts getting frustrated and ignores my corrections. So I walk circles with her. Even after an hour of doing nothing but walking circles, she doesn't feel any better. Just sweaty and tired. And lastly, when I'm holding her back she doesn't rear or anything. Just breathes heavy and side sets or jogs. She's never reared or bucked with me, even when she's extremely frustrated. The most she will do is side step or back up a bit then try to move forward again.
Try doing what I suggested above. Sounds like you were doing something a little bit different.
I would be rather anxious as well, if I got to do nothing but circles for an hour.
With what I suggested above, you are giving her a reward/release for walking nicely by letting her go straight
on a loose rein. She's only "punished" in the circle when she breaks gait. Rather than keeping her in a circle for hours on end, and never giving her the opportunity to walk straight.
Sure, she might only get to walk a few steps in a straight line before she breaks gait right away, but it is something. And you'll be able to build on it.
When you say you "put her back in the walk", how were you going about doing this? Using two hands on the reins?
I would definitely only be riding this mare alone, for the time being, until you make some progress with her issue. A lot of time when we ride with others, we sometimes don't school our horses like we know we should, because we don't want to "interrupt" the rest of the group.