The problem is that you have to unlearn the bad technique, not just learn good technique. That means you have to slow down your practicing to the point that you can force your body to move correctly. Then keep repeating and repeating until muscle memory kicks in. Only add speed when you are able to maintain good technique. Gradually speed up. It will take hard work and a lot of repetition, but it can be done.
Our daughter also needed to improve her shuttle run turns. We laid out lines in our garage for her to practice, but put them closer together than the actual test. That way she wasn't tiring herself out dashing back and forth. She focused on the turns themselves with only a couple of steps in between.
Good luck!