I've had a couple of young horses do very similar things when I am just starting them on the lunge line. This is what I do, and it has worked for me so far. Some of this you probably already know, though. With the ones learning I use a big area that is free of obstacles - I have a large roping arena and have been know to use the entire thing, end to end at the beginning. Anyways, I always make sure with the youngsters that I am looking at their flank, a good driving place. When I think they may try to stop and square up to me, I take my leading arm (if circling to the left, the left arm holding the lunge line) and extend it out to my side as far as possible, "leading" them forward and encouraging them to keep going. Then I point/swing my lunge whip with the right hand out behind, not at, but behind the hind end.
Now for why I like such a large area to work in. If she is circling to the left, you ask to canter using the exaggerated signals, and she stops and square up to you and pulls, I am automatically going to my right, trying to stay in that driving position, always exaggerating my arms as I described above. I may end up doing circles around them if they try to keep square to me, but eventually they will move out and away for you. As soon as they are moving back onto the circle, reward them, even if it's not at a canter. The main point is that they can't stop and square up. Eventually, she will canter or more often run off onto the circle. If so, I just let them keep going until they either break down into a trot on their own or they are doing a nice, easy canter and I ask them to break down. If they happen to stop and square, repeat the process.
I hope that helps a bit and makes sense. Feel free to ask any questions if it is confusing. Keep us updated on how it goes!
Edited to add: It takes some coordination to keep running to their side to stay in the driving position and not let yourself or her trip over the lunge line. Trust me, I've done it before
Also, as long as you don't think she will kick you, you don't have to be at the end of the lunge line when you are correcting her - just stay back and to the side, leading her with your arm.