My horse will do stuff that I swear there is absolutley NO indication that he is thinking of doing this, like spinning, and has dumped me multiple times.
All I can say is that you have to ride her in such a way as to keep her really occupied. Stay in the middle of the arena to start with and keep her really busy doing somehting that is constantly changeing. And the instant you see her attention wander, you must interupt that mental drift and get her back on you. 5 is very young for a warmblood. They mature slowly. I used to ride a 6 year old and he got a lot better by the time he was 7, but he used to bolt in the arena. If you Can stay with her, I might actually let her go forward and continue to ride her at a canter around and around. But only if you think you can stay with her.
It IS possible, that she is too much horse for you where you are now, and where she is now. Something to think about.