there are basically two ways you can approach this (more, but I know of these two). This is using direct reining, ok?
You can use the outside rein and outside leg to push her back onto the circle, as apparantly you are trying. You basically have to go so far as to counterbend the mare until she picks up her her inside shoulder, then return to your circle, but use the least amount of bend inward that you can. Think of steering her from her shoulders, instead of from her nose, and that means you are using the outside rein to lift her and move her around the arc of the circle
Way #2 is also direct reining. This is using the inside rein.
So , you apply the inside rein to get her to flex to the inside and follow a circle , say going left. But, either you overbend her, or she braces against that inside rein and ends up having her shoulder pop out to the outside and she starts to drift sideways. You take up MORE inside rein (becuase she is ignoring it), and you disengage her hindquarters. So, you lift that inside rein and sit back a little and she'll spiral around, with her back , inside leg stepping under her body.
You as much said, "NO. I will not allow you to ignore that rein and drift outward" it's more work for her. You start out your circle again, and if you ask her to follow that rein to the left and she drifts right, you disengage the hind end, and start again. But, if she starts to drift but then when you rais the inside rein, she quits that drift, you quit lifting the rein and ask her to "circle on".
It's a kind of corrective measure saying "follow this rein". YOu can back it up with outside leg as a support. If she blows through both, then disengageing her hind is like saying 'pay attention, you can't go on that track. Now let's start again"