First of all, rule out any physical reason for the behavior.
If this is only the second year showing for your horse, and only around 3 shows a year, she's still pretty green as far as showing experience goes. My first horse did a lot of acting up for the first couple of years of showing. Not as much as you're describing, but enough that I didn't feel comfortable riding him in the warmup area (an open field, we have really tiny shows
). He was a wonderfully quiet horse at home, and he did really mellow out at shows after a couple of years of experiencing them. I'm going through the same thing with my new horse as well. We go to a new place, and he starts whinnying and just not paying the kind of attention that he usually does. It can be an "info overload." If you can, try taking your horse to a show to just watch and get used to the atmosphere. Take the saddle, and ride around if you feel comfortable. Sort of going to the show, but not showing, if that makes sense.
Others can give far better information on dealing with her specific behaviors (the rearing does need to stop, that can really be dangerous if it gets out of control), but I think that a degree of behavioral change in such a different environment is to be expected, and with time and continued show exposure, she should calm down.
With the impatience standing tied, does she get tied up at all at home? I find it helpful to practice the aspects of showing that the horse will have to deal with as well as practice for my classes. Scout gets to practice standing tied to our trailer, eating from a haybag, etc. so it isn't totally foreign to him in a strange place.