Keep practicing them. Super important to keep your body tall. Allowing inside hand and a strong lower leg. Outside leg keeps them from bulging as well has an outside rein and an inside leg is used to keep their shape around the circle. Make sure your horses canter is powerful and collected, not just running around
Try ride in a dressage arena with letters ,circle at A and go outside 2 meters of P and V
Pull your outside rein to your hip bone and open your inside to your knee. Outside leg in back inside leg in front. Extreme. Inside. Leg.
Leg outside rein and help turn maybe
What is the problem? Is it not round , or do you just can not feel how large or small it must be ? does your horse go behind the vertical with his head during the circle ? Or does the inner reign feel like there is more weight in it during the circle ? Do you feel like you turn with the inner reign instead of turning with the outside reign? does your horse not bend ? 10 m circles are very difficult that's why they have 8 and 6 m circles even in third and fourth level Dressage tests because you can see if your horse is perfectly on the bit , and through.
Mark a 20 meter and 10 meter circle in your arena with cones(I put one cone 10 meters off the rail and another 20 meters) practice going around the circles until you know what each feels like. Eventually get rid of the cones and you should be able to do each circle.