Depending on how she was trained, she may just still be figuring out what you are asking her to do, when you do cue her; you stated that she was completely trained, but not to the way you ride her; she could be confused.
I would start stopping her IN the corners, so she learns that the corners are a 'good thing'; if she is indeed clausterphobic and that is why she's avoiding them, getting her to work off the rail, and then stopping to rest in a corner can help her understand that they are not a place in the ring to be afraid of, but a place she can 'rest'.
Also, if all you do is ring work, you may try to do more interesting things, like setting up an obstacle course, doing different cone patterns, going over ground poles, etc...switch things up so she isn't so bored with the ring work; some horses DO try to make things more interesting themselves, and that could be what she is trying to do by drifting away from the rail\corners. Go on some trail rides, or just do ground work some days.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."