That's great for your mustang, Widget! I don't really know anything about horses, and in fact, they kind of make me nervous. I'm afraid I may rate as a 6 or 7 on that scale.
Well being a Mustang who was caught out on the Nevada range she came with all the survival instincts that the domesticated breeds have pretty much lost over the generations. So training one is a bit different than say training a Quarter Horse. What might take the QH one session to learn and/or accept could take a Mustang 10 sessions or more. They just are so geared towards self preservation it can take them a bit to learn that humans aren't predators. And some never learn since the humans in their lives act too much like predators, usually without meaning to, that the horse never becomes really comfortable and willing to learn. One thing Mustangs teach us is patience. Training is on their time not ours. But if done right they are a horse that becomes a true partner. One who would move earth and sky for their owner.