It is no doubt balance issues. How long has she been under saddle?
Be patient and work in other areas that will build quality movement rather than allowing her to trot wildly in circles.
My friend, how coincidentally had an arab, too, had simialr issues. So, we worked out on the trails, on straight lines. I rode good old Mac in front and she followed. I would pick up the canter in front of her, and her horse would then fall into the canter off his own accord, to keep up, and then I would pull out of the canter. So, the time spent cantering was short, on a straight line and supported by an experienced horse in front. We did this a lot, and as time went by, her horse could hold a canter longer and longer and start taking it from a lead position, too.