First of all, as long as you think in terms of your horse being "bratty" you will be resentful about her behaviour because you will feel like she is doing it on purpose to annoy YOU. Your horse is not smart or wicked enough to have such an agenda. She has reasons for doing what she is doing, and they are valid for her. For example, she wants to go back to the way things were before, when she was sitting around and resting. Who wouldn't want that? That is not being a brat, that's being sensible.
But , of course, her job is to be a riding horse, so you need to change her mind. I only meant not to think in adversarial terms and that she is dissing you so you gotta get her back! Negative approach.
You just keep in mind what you want to be the outcome, and don't be dissuaded by her complaining about the change, ok?
YOu want her to move, right? Prefereably forward, right?
So, ask for forward. If she balks, ask her to turn one direction and go forward and yes, a smack on the bum. Think about breaking her out of her balk, so initially, if she goes to the side or backward or whatever, it doesn't matter, just so long as she moves.
Once she is moving then you try to get that movement to go where you want it.
If she throws her head, use one rein and bend her around and circle her. If she starts backing, back her in a circle using one rein only . If she starts to feel like she is going to rear, bend her around pretty tight.
I think that if these problems progress to the point where you feel unsafe and she is threatening to rear, then you must really do what it takes to bring in some professional training. Letting that habit develop is really bad, so very hard to break once it starts.