Hi Markie

This off the top of my head as I just read this and haven't given myself time to think about it, but a couple of suggestions anyway.

1. Do you have a Dually-halter? If so, they are great to start young horses with as you never have to disturb their mouths and the guidance you achieve is much clearer since the horse in turn doesn't have ti focus on his mouth and what is going on there. Do let your horse get familiar with how it works first! Also, you can haul at the reins a lot more without worry if you need to turn or stop more sharply, than you want to do with a bit.

2. With no visual of what it looks like when/where you are riding her, may I suggest the biggest distraction is on her back? (Nothing personal! To my way of thinking this goes for any unschooled horse with a rider on board.( ;) 
 It's one thing for her to follow your commands when you are on the ground and a completely different game when you are on her back; your head above hers and your weight/length to balance as well. It's hard enough on a straight line so turning might set her so much out of balance that she rather not if she can help it. 

I think you are on the right track with combining aids she already knows, like your voice. :) 
Maybe go with voice command first, if she knows that "by heart", then add the rein and follow with your body weight (including leg position) so that she has to shift if only to find her balance. Smallest attempt at doing what you ask must be rewarded with release and restoring your balance.
 
One way could be to ask one of the other riders to stay in front and you ask them to go left (or right) and then you follow their lead - alt. have someone lead or long line you, helping with the steering  until your mare is getting the gist of it. Be patient! This can take time. There is a lot of impressions for her to sort out. 

Come to think of it, my  horse was a lot more motivated to listen to the steering aids when I put poles (or trees in the forest) in his way like a slalom course.

Thanks for your advice. That’s a good idea about using poles and making a course for her. I will use your ideas. She is very young still so hoping just takes time and patience the dually halter i have but is getting to be too small unfortunately they don’t make them her size either 

Hi Markie. Your filly has lost the support of your body language which she has when you're doing ground work. Also, you may be less in control of your metabolism - your breathing & heart rate/adrenaline level ( after all, riding is more exciting )! So, I suggest you practice keeping yourself really calm & focused, like you probably are during ground work because that was successful. Kicki has great advice too. I'm puzzled though; what kind of horse is your filly? The blue dually halter is huge! It's for shire horses or Clydesdales. I'm guessing your filly is outgrowing a smaller dually. You can add extra adjustment by making more holes. These can be sealed ( to keep them easy to use & stop them wearing into odd shapes ) if you CAREFULLY heat a metal tool ( an old, medium sized screwdriver would do ) & burn/melt the new holes to give them a structure. Please don't post back that you needed a trip to hospital or burnt down the house - resize halter with great care! Looking forward to hearing how you & your filly get on. Cheers, Jo.

Hi. Subsequent thought. As you work in the field with the rest of her herd perhaps she would find it easier to work whilst returning to her friends. Lead or long rein away from the herd, mount & ride back in a direction she finds logical, back to her pals. That way, she'll see virtue in having a rider & be more inclined to cooperate. Cheers, Jo.

Lol. I hopefully will not burn down the house. Thanks for your input. She’s a Percheron. A big Percheron. The halter still fits right now but that won’t last for Long it’s way too small on my other Percheron. I will give it a go riding back to the herd!