5 year old aggressive unbroke horse

Question
I've been around horses since I was 4.  I could catch the uncatchable.  I've trained my own horse before, but am having problems with one that I've just recently purchased.  She's a 5 year old quarter horse, and hasn't had a thing done to her.  Basically she just ran wild with her siblings.  I've been spending hours with her, talking to her, feeding her treats, and can touch her face, and she'll stand for me to walk up to her.  I've been round penning her lately, to work her a bit, give her some exercise, and she's been quite submissive.  Today I tried round penning her, and she jumped the gate.  I was taught to not let her "beat me" or win.  So I put her back into the pen, and just asked her to walk and trot around.  Suddenly, she turned aggressive towards me.  She tried kicking, but then would relax and submit, if you can call it that.  (lack of the correct word).  Then she suddenly tensed up, lifted her head, ears pricked forward, and started to run at me.  Nothing spooked her, and there was nothing around for her to shy at.  I was baffled.  I hopped the fence, and she went to the other side.  But as soon as I climbed the fence again, she came at me.  I'm baffled at this aggression, as she was responding so well to me.  I'm really hoping that I haven't ruined her, but I'm not sure what my actions were that caused this.  Is there something that I could have done to cause this?  And now I'm afraid that she'll keep trying this everytime I enter the round pen.  Do you have any suggestions as to why she might be doing this?  And do you have any suggestions as to what I can do to help her over come this problem?  

Answer
Jenn,

Remember you're dealing with a teenager here. She's hit the rebellious stage which is where the aggressivenss is coming from.  She's been a "feral" horse all her life and now you are asking her to submit and do some work.  The best you can do is let her know that this is unacceptable behavior on her part.  If that means screaming and yelling and waving whatever be it a lunge whip, a rope, whatever to wave and letting her know that this is not gonna happen.   Think about horses in a herd.  There's a dominant horse, usually a mare in a mixed herd (which should be you in relations with horses) and she's challenging you.  You'll have to watch your step, maybe station a few people around the pen to help keep her from jumping out.  Mainly get her moving and keep her moving as long as she shows aggression.  See what she does each time you let her stop. Depending upon how determined she is be prepared to take as long as it needs to.  If it takes more than one lesson the do it.  I've had this happen once with another horse.  You have to get their respect.  Once you have that then continue on with your work.  I suspect she is a pretty smart horse so make sure you keep up with her and keep her interested during her training.  Don't let her get bored.  Make it fun.  

Let me know how you are doing with her.