It sounds like a respect issue. Try using your body language more. You can also keep gently tapping her with the longe whip and keep a steady rythm until she moves out and then instantly stop.
To help build her respect you can play the "yo-yo" game. Have her on a lead rope facing you. Then send energy down the rope by shaking it. Start little and get bigger as you need to. She should back-up. Then have her come back to you. Keep doing that a few times, but not so many that she gets bored.
You can also see if you can move her on the ground by just lightly pushing her to yield to you. The instant she yields stop the pressure you are putting on her.
It takes a lot of young horses a long time to master the concept of lounging so be patient. When she tries to cut back in send her back out with your body language. Lean forward and kinda crouch and stare her down. She'll probably go out on her own, but you may need to encourage her with the whip. Crack it and wave it around, but don't be abusive with her. You may have to tap her, or give her a smart smack, but nothing too severe.
I hope that helps