Make sure you are using the inside leg to outside hand. Inside hand doesn’t keep the horse in the circle it just creates the bend. The horse is balanced and controled by the inside leg to outside hand. I also like the spirals mentioned above.
Also some circles in canter, spiral in and out - push through the unbalanced canter (and give confidence to the horse to know that it can carry through) - and then leave the circle onto a straight. Cantering down 3/4 lines & pole work as well will help balance and straightness.
Lots of transitions! This is usually because the horse is not strong enough to hold itself up. Picking up a canter for 3 strides then back down to a trot, then slowly extending the canter makes them use the hind end and build muscle. Also canter poles under saddle or on the lunge really help to build muscle, and make them support their own canter! Hope this helps!!