First, I would recommend you to go back to a plain old snaffle bit if you aren't in one already. Teach her how to flex on the ground, put a little bit of tension on one side of the bit and wait until she is giving and is not moving her feet. She will most likely walk in little circles for a while before coming to a stop, just stay with her and keep light pressure. As soon as she gives, give back. Work on this on both sides.
This will teach her to be lighter in the mouth, it will get rid of that fight and shaking of her head. It is also an emergency brake so to speak. It's much easier for a horse to brace on both reins instead of just one.
Another thing she needs to know is how to disengage her hind quarters. Start with her in a flexed position. Put your hand ( with the leadrope) in a fist and lightly press where your leg would go with only a couple ounces of pressure. Then take a crop and lightly tap her on her hindquarters, building intensity slowly until she moves her butt around. You are only using a couple ounces with your one hand because you want her to be light off your leg. The whip is just an aid to reinforce your 'leg'. Get this consistent on both sides. You want to make sure tht the leg that is moving away is crossing over the front of the other back leg. If if doesn't keep pressure on until it does.
All of this sounds very basic I know, but you need a solution to your problem before you go diving in.
Then you can incorporate these things under saddle in a controlled area like the round pen or an arena. At this point i'd carry a dressage whip just to ensure she'll be getting off your leg.
Mount with her flexed. Then flex the other way making sure that is good. Next is to disengage her both directions. Then ask her to walk off. At this point you aren't steering. Just let her wander around on a loose rein and bend her to a stop every so often, making sure she disengages as well. If at any point you feel her tense up or want to go faster just bend her to a stop.
All of this is reinforcing that she goes when you ask and stops when you ask in a safe way.
When that is good ask for her to gait (if she has multiple gaits, whichever one is slowest) and repeat.
You will find that she will be drawn to one area of the arena which is fine. Next is working on getting her to NOT want to be there. Let her wander to wherever she wants, then bend her around and disengage her. Use your crop to tap her on the butt and keep her going around for a few circles. Annoy her, make that area uncomfortable. Then when her head is pointed away from that area let her back out into whatever speed you were at. Every time she drifts over to the same area a lot, do this. Make the wrong thing hard and the right thing easy. In this case the area in the arena closest to the barn is hard and away is easy.
When she wanders away from that area bend her to a stop and let her rest for a couple minutes. Be consistent in that away if easier work and closer to the barn is harder.
Up to this point, if you work with her every day and have decent timing this should be over the course of 3-4 days. You might be thinking that this is longer than you'd want it to take but look at the bigger picture. You've already been riding an out of control horse for how long, so what is a few days to get her safe? Keep in mind some horses need longer, some need less. The important thing is not to rush.
ONLY when she has that concept grasped do you actually steer and this is for your own safety. Barn sour horses can be very difficult. Some bolt, other rear, other buck. This is because they want to be in an area they want to be in that you don't. So you need to get that part worked on first.
The first steering you do with only be corrections. Again you'll be on a loose rein and this works best at a trot when teaching it ( or in your case whatever gait she does). Let her wander around, if she veers off to the right, steer her off to the left. Don't just get her back on the line you were on, over correct and go way the other direction. If you veers off to the left, go right.
You need to make sure you CAN steer before you go picking exactly where you want to go. If she still picks to be in one particular area go back and annoy her by disengaging and tap tap tapping with the crop. When she is pointing away let her out. Then when she chooses to be away, bend her around just till she walks. Make it work where she wants to be and rest away.
BIG NOTE: If at any point she blows you off and doesn't give to the bit, hold the pressure. Do not get more firm as her first reaction will be do pull harder against you. Wait for a slight give. Whatever step you were at, then go back and work on your flexing for a couple minutes to get her soft again.
When she is consistent good in the arena then you can go back out on the trail b/c by that point you will have a solid foundation on her. Keep in mind what you did in the arena however. I would work her a bit in the arena first then go out on a short relaxing ride down the trails. When you get back, work her in the arena some more.
If she tries to race back, bend her to a stop, disengage her then ask her to just stand again. Rinse and repeat every singe time she gets anxious and or tries to speed up. Staying patient is very important.
Good luck :)