Breaking racehorse of habit?

I have a new thoroughbred filly, she'll be 4 in the spring and she's pretty fresh from the track. She's doing extremely good, but when we're cantering she swing her rear end towards the inside so she almost looks like she cantering side ways. She isn't responding at all to leg pressure and I can't really think of how else to get rid of this habit. I was thinking lunging would help, because it'd be pretty hard for her to keep her rump to the inside when she's doing a small circle.