Phase 2 Bonus: Tipping point

The mare had been taught to suppress negative stimuli until they built into "sudden" outbursts of fear, which was a point of frustration for her owner, who didn't make any such connection and marked it off to her just being like that for no good reason. They even tried training it out of her, but ended up only making things worse and the mare grew more insecure with every training session.
Despite that, her owner wanted to take her to local shows, as that had always been the intention. Beforehand even joking with friends about how the mare was scared of the most stupid and random of things and hoping she wouldn't be like that in the show ring.
They didn't even get to the show grounds though. The day of, they tried loading the mare into a trailer for the first time in ages. They were short on time, yet the mare just did not want to go in. Never before had she offered any such refusal, always giving into pressure rather easily. This time nothing could move her. Her owner was mad, the mare was scared and stressed, it was an all around bad situation. After this the mare was dubbed as stubborn and her reaction brushed off as such. Her owner saw it as such a dumb, senseless thing to refuse.
Following that day they attempted to re-touch her trailer training.
They made no progress however, the owner tried to push and pressure her into it while she was getting more terrified and confused.
The stress of her frustrated owner pushing her, the quickly intensifying fear of the trailer and recent experience with it combined with her already lingering anxiety were about to send her flying into a panic.
Whips and sticks with bags on them were waved behind her, ropes around her back end to push her in, a halter with chains tugging painfully at her face as she was dragged in, centimeter for centimeter.
Finally the doors slammed shut. Locking her in this suffocating place.
She freaked out, positively lost it. She was frantic. The more she panicked the more she realized she was trapped in this terrifying space and the more she panicked the more horrid her situation became.

Her owner had come to a point where they were convinced she just needed to learn this lesson and were self assured in waiting for her to calm down and realize that this was something she "couldn't get out of by throwing a fit".

And eventually she was tired out. It reassured her owner they were right.
She was shaking, terrified, soaked in sweat and heaving.
The mare staggered out of the trailer, appearing just as calm to her owner as they had wanted. Her owners' glee at this success didn't last long though. She refused to go into the barn. After pushing and prodding her to go in just as they'd done for the trailer, the mare couldn't offer any more resistance and just collapsed.
Even after recovering a little she panicked when dragged beyond the threshold. Nothing could calm her down, no amount of food could lure her, no pressure could get her in anymore. Then she wouldn't go into the covered arena,
or the shady pasture, which already had been her owners last choice, worrying more about being able to catch her again than anything else.
Ultimately she was put in a paddock to calm down. Her owner figured they could wait it out and in a couple of days she'd stop being like this.
She however could not rest or relax. The owner's regular, ever more frustrated attempts at getting her back into the barn led to more panicked breakouts and an ever more stressed horse. She was never at ease, only vaguely yet inescapably anxious. A new terror always lurked nearby, not even an open shelter or a trees canopy provided any form of safety. It was the opposite, they had become scary and foreboding to her.

The mare's world no longer was a safe place.

After a while of this the barn's owner wanted the paddock freed up, the vet bills (the vets had not found any physical issue) soared and the mare had become less easy to handle. She wasn't aggressive or disregarding her handler, but she was always on edge and spooked at anything, turning her into a headache to her owner. Eventually they concluded this was all a behavioral issue with no real reason behind it. Deep down they likely knew what they'd done wrong, even if it was vague, and were scared of their own failure, the idea of having to admit fault.
They felt urged to get her off their hands and realized selling the mare would only be more work, so they ultimately pushed her off to the treating vet. The vets whose advice had met deaf ears so far- Who had a general idea of the mares' struggles and where they stemmed from. The vet quickly organized for her to be brought to a rescue they knew and trusted, providing the mare with a soft landing and the chance at a different life.