Hi, Hanney, and welcome to the forum!
The goal is really for horse to learn to maintain its own balance, rather than you doing it for them. (But I suspect you know that
I also would not work on 15 or 20 meter circles just yet - if she can't maintain a good, balanced canter to the right on a straight line, small circles are just going to be frustrating for both of you.
The first thing I would suggest is to get her fitter. If you have access to some trails or open land, do some long trot sets out in the open and hill work.
Next step would be to work on improving her canter on the lunge line. I would lunge her in side reins to encourage her to use herself correctly, and I would spend twice as much time working to the right as to the left.
When I was back in the ring again with my fitter, more supple horse, I would do a lot of transitions in and out of canter - canter/slow trot, slow trot/canter, canter/walk, walk/canter, halt/canter, canter/halt. My goal would be to ride a transition BEFORE she loses the good quality canter and tears around on her forehand. You should then be able to gradually increase the amount of time she spends in canter.
I wouldn't add the circles back in to your routine until she can maintain a balanced canter for one entire circuit of your ring.