Can you pony one horse and ride another? You could even switch off whose ponying and or point out that her horses really do need exercise so you will pony one and ride yours, and she should pony another to fully make use of your time. She's 17, maybe responsible enough to make sure horses are fed, cleaned, watered, and handle minor squables, but probably not always fair and diplomatic. If you really want to fight fire with fire, insist on riding your horse, if she refuses to go, go anyways (if your safe riding alone on this type of ride), then the next time you know she is going to be riding on the road, tack your horse (in his stall if you really don't want to tip her off) and just insist you ride with her. She can't control the ten foot radius around her and say your can't ride there. Just leave when she does, then what choice does she have anymore. Sounds like you are learning from her and kind of a working student, and sometimes that requires doing what the trainer wants done, not what you want done. I woud try ponying first though, and you can politely insist that you trade off or both pony a horse to make it fair.
Troubled TB ~"A thorn by any other name will prick just as deep." @-'--,---