Smrobs, you are one amazing young woman...and you are right on.
I am one all for desenstizing to anything and everything I can, but when it comes down to it, when it comes to getting a horse used to 'stuff' in general, you just have to get on and ride, because doing desensitization from the ground is entirely different than being in the saddle; yes, getting them used to stuff like scarey bags CAN help alot, but it's riding that makes the horse 'broke', trained, what ever you want to call it!
I never 'introduced' my mare to town riding, really, other than just working her in the safety of the 'arena' at the barn, and then going out into the streets when she was relaxed and somewhat 'tired'. And guess what? She has taken to it like she's done it her entire life; you can't get scarier than busy underpasses...both above and below, and she's never flinched. A confident rider, regardless of how green the horse is, is what gets a horse well broke...and of course all those wet saddle blankets!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."