My horse anticipates the canter. any tips?

i have a standardbred mare that im having a little trouble with. when i ask her to canter shes good but if i want to go back to a trot she does but then in the corners she goes back into it. so ive been stopping her and backing her up and continuing with my routine. circles are a little hard in my arena because she can bend perfectly so she just drifts even with an outside rein + leg. also, if i gather my reins up she anticipates either the trot or canter and starts to like jig around so i go back to stop her but sec later shes back at jigging again. i have lunged her before and it doesnt work. she doesnt get any high energy food i would just like her to wait for me to ask her to canter rather than having her jug around for 10min. any tips?