Hi, Welcome to the Horse Forum.
This, again, is not all that unusual. Blue Spark has already told you what your options are. My choice would be to get some local trainer or very aggressive rider to ride her through it -- first in THEIR arena and then in yours. Then, you should put one or two rides on her right after she has ridden well for them.
I would do it this way because she has been allowed to get 'arena sour' with you and she knows it. If you ever hit some small issue on the trail, this behavior of 'bluffing you out' will reappear over that issue and your trail horse will do the same thing she has learned to do in the arena.
As I have said many times "Horses are creatures of habit!" At some point, in an arena, she bluffed you out, stalled out and and refused to work for you and you let her get by with it. It taught her that she is in charge in the arena. This happens so easily, that we do most of our training out in big pastures and out on the trail. That way, if someone is working on a problem, am arena does not become a 'place of conflict' in a horse's mind.
No matter what we are training or working on, we frequently go into the arena at the end of a long ride and finish there. We lope two or three circles, ride to the far end or corner, stop and rest there for a few minutes, dismount there, loosen the girth and lead the horse out. It keeps horses attitudes good about arenas.
If you get someone to 'fix' your horse, then I would start doing the same thing with her. Always end on your terms. Be very careful that she does not give you a signal and you obey her. They learn very quickly when and where they are in charge. Always end a ride with you firmly in charge. Always quit while you are ahead. NEVER, NEVER let her determine what you do. Many horses get really good at 'training people'. Yours has mastered it in the arena.
Bottom line -- This is NOT due to a hole in her training (although she could have several); This is strictly the result of a hole in your riding. Your lack of confidence and assertiveness the very first time she threatened you is what started this. Then, she capitalized on it. That very first time you should have MADE her work a few round in the arena before leaving. Now, I would not suggest that you use whip or spurs because she may put up a lot bigger fight than you are able to ride through. Let someone younger and used to doing this tackle the job. When I was able, I used to help a lot of people through problems just like this. Once someone gets her 'ironed out' for you, it is the psychology of horse thinking that you need to address. They are not unpredictable. They are the most predictable animal I have ever been around. People just need to learn how they 'think' -- how they react and how they respond.
Last edited by Cherie; 10-22-2013 at 09:05 AM.