Sometimes you can make your circles better by not working on circles! Work on squares, 90 degree turns, and diagonal lines for a while, and maybe some nose to the wall leg yield. Then come back to your circles, after you have worked on better use of and response to your outside turning aids.
Check for self carriage often throughout your work. Each time you do, you will get clear and accurate feedback as to whether or not your horse is carrying himself properly.
"Remember, the conversation between you and your horse must never be dull or inert. It should be, 'Ask, receive, give. Ask, receive, give.' Ask with your body and legs; receive through your body into your hands; give primarily with the hands, but also with your body and
Remember that whatever you are used to usually feels right to you. You will often have to step out of your comfort zone if you wish to improve.
“Thoroughbreds tend to thrive on physical movement, and they are, like all horses, hypnotized by their own rhythms. Repetition of success creates a sense of peace.” ~ Steuart Pittman