I agree with iridehorses.
Another thought... was she exposed to the round pen in a bad way at any thime in her life?????
My mare was scared to death of round pens. But she had a very good reason.... a few years back when a lettle girl owned her, she was leading Emma (the mare) through a round pen bow gate and the saddle got caught on the gate and.... long story short, the round pen was tore up, Emma was toreup ad the saddle was ripped to pieces. It took me a long time just to get my mare to go through gates camly. She went from freaking out going trough a 20 ft opening to calmy walking through a 4 ft bow gate. She also lives in the round pen that I work her in. That seemed to help alot as well.
Like I said... great advice iridehorses