I really like what mayfieldk said about slowing a lope down, and asking for the leads. Here's a slight variation, though, that helped my horses quite a bit, but your horse has to know how to turn on the forehand and hindend very solidly. When loping, especially when refining, I want the hind end in some (varies between horses), that shoulder up (I suspect yours is dropping since she speeds up on circles), and the head straight. If my horse starts to speed up, I force them into a stop with a counter-bended turn on the hind end. So if I'm loping left, I pick up my left rein, bringing it towards my shoulder and forcing that shoulder over to the right. I might use a little left leg, but it's almost all rein. Ideally, this lasts for 180 turn, but if needed to get that shoulder up and moving away to the right, it might be for a longer period. But that shoulder needs to lead the turn to the right, with the head and neck staying bent slightly to the left. Once I get that shoulder moving over, I keep the front 1/2 of the body in the same position, but switch to using my right leg to move those hips over to the left, getting them off of my leg. Once they are off my leg, I ask them to lope off staying in that same position (doesn't matter what direction they lope off in - correct lead, counter canter, just lope off without trot strides). Hip left, shoulder up. I let them go a couple of strides in a good rhythm, then slowly release them and let them hold themselves in that position. If they lose the position or speed, I repeat the above sequence.
Another thing I really love doing is counter-cantering, building up to large amounts of time. If done correctly, it will help keep the shoulders up, and the horse moving off of your leg. Just make sure that the horse is driving from behind, and keeping a good strong rhythm and impulsion. It also really helps create the muscle strength to hold a lope for WP.
If any of this confuses you, let me know and I'll try to explain it another way.