Well here's the good news. While she make be a chicken out in the hay fields, she's likely to be very different bordering on brave when eventing. My TB also has moments when we attempt to trail ride or ride in fields alone, but he's a machine when we are eventing. I guess it's because there are other horses around... and it's fun.
Now what to do about it? Having also been on that snorting, rigid monster who heart you can feel beating through your legs, my first step would be getting her used to it without you being in danger. This being a mare and not a gelding, she's not going to back down out of her funk as quickly as a gelding would and there are only so many one rein stops and bolts you can prevent before finally landing on the ground. I would take her out there on a longe line first with the goal of her working calmly in circles around you. Expect a dizzying gallop at first, but let it play out until she figures out she's going nowhere fast and starts to relax. The goal is relaxation. You know her best, so figure out whether you need to just walk her to not get her worked up, or use the upper gaits to keep her mind focused on working and not the monsters. Only do this when you have tons of time. You cannot stop until she is calm or else you've accomplished nothing.
Once she's listening to you on the ground, try it under saddle. If you can feel her "moments" coming, before she gets too rigid and don't want to move, put her to work so she focuses on you instead of the monsters. Also, if you can figure out what part of it is bluff versus real fear, try to call her bluff when you think she is faking by making her work. If there are any objects on the ground out there, walk or trot her over them. Keep her as engaged as possible. Make it fun for her.
Stay safe and good luck!
You just have to see your distance...you don't have to like it.