I'm not the lunging expert but when I used to lunge my friend's 2yo for her he used to turn in constantly and I was always told to bring the whip infront of him (if that makes sense) and crack it near infront of his chest to teach him that the middle of the circle is basically a place where the whip will get you, so he won't want to go there anymore. Also, if possible get a little more speed up before the point where she stops so she won't get chance to stop.