Training a 3 yr. old

Question
Hi.
My name is Amanda and for the last six months I have been assigned to the task of training my trainers 3 yr. dutch warmblood/thoroughbred cross filly. The filly was born on the property where she has remained ever since last month when I brought her to my home (home from college for the summer), which was only 2 hours away, where i could continue her training. She had been doing walk, trot, canter in the round pen, all voice command, and I had sat on her a hand full of times , doing walk and trot also in the round pen. When i brought her home I started working with her in an oversized dressage arena to give her more room and to begin teaching her steering, ect. The filly is super quiet on the ground and will stand, tie, load, clip, you name it she will do it with no questions ask. Even when I mounted her for the first time she never questioned anything.
Recently I have been working to keep her in a working trot and some problems have been occuring.
*The first time I ever trotted with her in the round pen she got off balanced and slammed on the breaks, putting her head between her legs and sent me flying over her head(she never really bucked or anything). I got back up and didn't get on to her about it because I figured she had just reacted to the shifting in my body weight at the trot (which i weigh about 110 pounds so i dont think i am to heavy for her). We had no more incidences until i brought her home, when she repeated the same thing a week or two after we had been working in the arena. Once again i did not get on to her because it didnt seem like she did it to be bad, but just got confused ( although this time she did hop with the hind end a little, once again sticking her head between her knees and sending my flying again). I got back up and got back on and continued trotting as if nothing happend. Neither time was she upset by me falling off and both times stood there and let me remount without any issues. (this was about 2 weeks ago).
* Yesterday I had just got on and was walking her on the outside rail in the ring and she jumped up in the air and stuck her head between her legs again and sent me flying, except this time when I got back up she continued jumping in the air, head still between her legs- i mean it looked like something from a rodeo and she was humping up like i had a bucking strap on her or something. I calmed her down, remounted, walked her around once and had to call it a day because a storm was coming ( once again she had no problem with me remounting). This morning i went out to ride her with a game plan, to keep her head up so I could atleast have some control if she went to bucking again. We did some walking and then started on our trotting exericse. Once she figured out i was not going to let her get her head down she tried to stop trotting but i encourage her on my clicking and adding a little pressure with my legs and reassuring her with my voice (which she is very responsive to, when i am riding and when she is on the lounge line). She tried to stop a couple more times, and when i again asked her to move on she jumped up in the air (this time without her head between her knees b/c i had a hold of it) and took off running toward the end of the arena. I slowed her down but she continued jumping in the air and went to try to run again, but before she could a jumped off her landing on my feet. I sent her around the arena several times, then tied her up in her stall for 1/2 hour before I had to untie her, because i had to get to work.
- I have been riding for 10 years and honestly have ridden some problem horses in my time. But, the thing is this filly never seems to do it to be mean, she actually will come and put her head in my hands when I get upset with her and never gives any indications she is unhappy while being ridden, in fact she will stick her head out her stall window and whinny for me, until it is her turn to be ridden, which is why I am having such a hard time. The last thing i want to do is over react and "break her" down, but i also dont want her to think that this is ok.
** I know i have written alot, but i just wanted you to kind of see where i am coming from and ask your advice, because i was really hoping to start working on cantering soon, but until we get this problem resolved I dont want to move forward.
If you could please give me your opinion it would be such a big help.
Thanks so much!
Happy Trails~'
Amanda  

Answer
Hi Amanda, thanks for your question.

First off, I'm going to say that when a horse shows any sort of unfavorable behaviour it doesn't not always mean they're trying to be bad... but the behaviour is still unnacceptable and the horse must be taught that it is not allowed.

It almost sounds to me that your filly has discovered something fun to do while being ridden... play. While its nice to  know that they are comfortable with us, playing should wait until they're in the paddock/pasture. We need to get through to them fast when this behaviour shows up, because while they may not mean to hurt someone, they can and will if they act silly while being ridden or handled.

I don't know if you have access to a roundpen anymore, but ideally this is what I would do:

Put her back in the roundpen for a few rides. Make sure you lunge her at all gaits before mounting, practice your bending and backing too. Once you get on her, stay in the roundpen. Work on getting her to go forward at all three gaits... if you have someone who can lend you a hand, it is really handy to have someone else in the middle of the pen while you ride. This way, if she stops, or trys to buck, or anything of that sort, you can concentrate on keeping your balance and holding her head up while the person in the center can help you to drive her forward... she can't do much while she's in forward motion, so they safest thing to do is keep her moving.

It really is important that you are able to walk, trot, and canter her in the enclosure of a roundpen, as well as have a first stop and steering, before you take her into a larger area. Young horses need security, and plopping them out into a large arena takes that way from them... so try to keep things simple.

Once you have her under control in the roundpen, take her back outside. Make sure you canter her outdoors too... or its going to be a big fight later on when you have been ignoring the canter for so long. You don't need to do lots, just make sure each ride you canter her even just one lap each direction, then go back to your trotting excersizes.

If you do not have access to a roundpen, it is important that you work her through her misbehaving... if you have a ride where she tries to buck or get rid of you in any way, you need to drive her forward, start doing small circles and then moving forward again, etc. But do NOT stop her... or she wins. You have to keep her going. If you are afraid that she'll just buck, make her circle circle circle. Make sure you aren't just pivoting her, keep all of her feet moving, but in a circular motion... she'll start learning that bucking and bolting makes extra work for her and its easier to just do as you ask.

I hope this has helped you a bit. Please feel free to contact me again if I missed something or you need further help.

Good luck!