I have found that putting either of mine on a longe line tends to make them worse (possibly because it wakes them up?). Each horse is an individual. Can you try asking her to trot on a diagonal, or a figure 8? Something a little different from your normal routine, so she has to think instead of just getting it over with as quickly as possible? We do a lot of small circles, in different places, some lateral work and some straight line (down the center of the ring), including asking for a halt at various times for no good reason (horse’s perspective). It doesn’t always work the way I would like it to, but that is my fault, and when it does work there is a huge difference, because we have to pay attention to what we are doing.
It is never the horse's fault