Need help keeping horse under control in arena without being rough :/?

If you don't want to be rough with her and she is really barn sour, it might be a good idea to start her out with another horse in the arena. This might comfort her and help her settle down. However, you shouldn't rely on having another horse with her all the time. Ride her with someone else until she settles and learns to relax and enjoy riding. Then slowly wean her off needing company after you've made some progress. You are right that you probably don't want to be too rough with her that this point. She's new to riding and beating on her will probably just stress her out even more. Do your best to make her time in the arena fun for her.