Help on Horse Training?

I have a black and white Tobiano APHA yearling Filly. Her registered name is Tuff Zippin Lux, and I'd like some advice to go about training her. She was imprinted at birth and she's practically bomb proof already, she bathes, loads, clips. You can lean over her back and put a surcingle on her with no problem at all. But when she goes to lounge she gets so stubborn! It's like once you get the lounge line on her and get her in the ring she's a whole different horse. She leads perfect and everything but I can't for the life of me figure out why she won't lounge quietly. She doesn't buck or rear, those aren't the problem, she just either plants her feet and pulls back on the lounge line or she just canters wildly in circles and refuses to listen to my body language and my commands. I've tried multiple methods and none seem to be working. She'll do other things for me, she'll give way to me, and when I'm leading her she listens and does everything I want her to. Any suggestions on how to fix this? Also I can't discipline her, I've tried giving her a firm hand and a gentle hand and when I give her a gentle hand she ignores me and a firm hand just increases her stubborn streak tenfold. Any advice on how to handle this kind of personality would be great, thanks!