I have been waiting to see what kind of bit you currently have her in as well, but did some thinking and wanted to give you what both myself and my two sister trainers typically do when this crops up in our clients horses.
Typically when we have a young horse do something like this, it's because they are either bored, tired, unbalanced, or have gotten into a habit.
The suggestions already put forth by the others are, as usual, excllent idea's, and I would run through the gauntlet of them, one at a time, on seperate days, and find what works. If she is truly bolting and not just being "heavy", then IMO, restart her totally. As if she never had had a halter on even. That tends to be a trust issue, and can be Very dangerous as others have mentioned. However it sounds like she just hates using her hind end, and has discoverd she's bigger then you, and has gotten into the habit of leaning on her front endwith you as a crutch. (Which stinks.)
First off, get the nubby spurs, they will be Very useful for surprising her foward or off your leg, (and therefore out of the shoulder slid )
The half halt exercise outlined above by Whitney, is a wonderful one, but make sure when you half halt that you belly button come back to meet your spinal cord, (and when I say this I don't mean suck it in like your getting back into your old skinny jeans, (Your Shoulders Should Not rise when you do this, if they do, you are doing it wrong) instead think of your stomach comming "back", it should feel a bit like you are sinking down in your saddle if done properly, to check, sit on your hands in your saddle with someone holding your mare, and try different ways of pulling in your stomach muscles, until you feel your saet bones sink into your hands. To get an idea of what this weight Should feel like, while sitting on your hands, make yourself laugh, your diapharam will push those muscles down, and increase the weight. This is what you wantto do, minus the goofy cackling up on your horse.)
While doing this, make sure your lower back is Very still,(Imangin wearing a corset, a Very tight one) and that your inner thighs are also "freezing".
You want to convey No motion at all so as to Stop the motion in her back. Remember, despite your size, you are Sitting on her spinal colum, and if you truly freeze, so will she have too. (So think hips, back, stomach, thighs, shoulders, frozen. And look Up not down.) And when she does, Reward like crazy, happy silly voice, pats, maybe a walk doing nothing for a second, Anything to drive home that listening to your body was a great idea on her part.
The oppsite is true as well, when you walk think of all the motion going through your hips, not your stomach, and start to get her used to listning to your bodys signals and not so much your hands. After she stops easily at the walk with a continued half halt, (And make sure she stops with her head slightly on the bit, Not up in the air with her back all stung out behind her, you are trying to get her to use herself, not your hands. )
Then proceed to the trot work. Sitting trot, with about 5-10 steps trot, then walk then halt. Shorten this exercise down till you can go 3 steps trot, walk, halt, then trot off again. Then mix it up, keep her guessing. Trot 5 steps, halt, turn, trot 3 steps walk, etc. Make sure your outside leg is right there to catch that shoulder when it goes sliding, a smart tap foward of the girth should open her eyes to your new found friend the spur.
The idea is to keep her in her hind end. Your reins should be realitivly loose once she is listining to your body, in the beginning you may need a shorter one for both your safty, and to get your point accross.
All reins are is a place for your hands to go, since they'd look goofy in your pockets.
Your body does the real work, and here, your height may auctually help as short bodied ppl tend to have stronger and shorter backs, which makes it easier to immbolize.
If she goes to turn to soon or evade, use your thighs as a channel to keep her straight, and when she goes to pull, either just give her one, or none, or better yet, half halt/stop her totally, so that she can't fall foward into her forhand and yank on you.
Could you post a picture of her? If she's naturally a downhill horse,(For example a bulldog or cow bred QH, they tend to be butt high, and built for facing off with cows, the neck ties in lower, and they tend to have very big forearms to support the build) your work may be harder, but if she's an uphill built horse, (think Arab, super high neck set, flat croup, relativily leggy and evenly muscled on both the foreend and hindquarters) you shouln't have a huge problem once she gets the gist of things.
Whew that's long. Sorry for the length, and if I missed something or wasn't clear enough on something you can pm me or ask here. Hope this helps, we have had that problem alot with many of out clients young stock, (the PMU foals are very popular down here as well) and this has worked very well on the babies who had your mares habit.
Whitney
P.S >All my horses, from school ponies and up, will stop dead at a 'frozen' body, turn with your slightest direction from your body, and collect to a loose rein. Check out Mary Wanless, and Jean Luc Cornelle. I have ridden in lots and lots of Trainers clinics with both, and they have excllent books out there for problem youngsters.
Also I will try and get some pictures in the next day or two to illustrate if you or anyone else would like.
Other people have ridden her (all who weigh more then me), and all say she really reacts to weight, so I'm a bit lacking on this part. I don't want to get into a pulling match with her. Any ideas so I can just have some fun with her until she leaves? I wasn't sure whether using a hackamore or other bitless bridle would help??? Anyone have any suggestings? Sorry this is long...
