Horse in a Rush

tight openings are scary indeed.
When a horse is frightened by a sound sight or sensation less significant stimuli are ignored. Thats why when a horse is in allout flight mode hes tuned out to almost everything else even so far as to collide with a fence or scramble until injured to be free of restraint. A handlers tugs on the lead are unlikely to interrupt a horse dashing through his stall door or flying backwards off the trailer ramp.
10 Slowdown Principles
1. Note a small acceleration and nip it in the bud. Do you notice your horses energy build as you position for a transition in a flat class? Settle before he bolts as the other horses in the ring canter by. Look for the point of arousal the pricked ears raised head and added animation in your horses stride preparing for flight. Doing this requires good timing and the ability to read a horse. Reading the thought of flight response allows you to decide to gear down turn another direction or stop before the horse rushes.
2. Maintain the rhythm. Selfcarriage describes a horse staying within an imaginary box when the riders aids are relaxed without having to be held there. If the horse is hurrying he will be showing some degree of flight response. Aim to lengthen not quicken the stride. The key to a relaxed horse is in keeping the same tempo or drumbeat of the legs while gradually stretching the stride. Longer not quicker steps are the key to success in most equestrian disciplines.
3. Dont trap a horse with your aids. Conflicting aids create confusion. When a horse becomes confused its first reaction is usually tension. This tension makes the horse inclined to run away from the stressful situation. The more conflicted the horse becomes the greater his instinct to run or shy away. 
As a green rider my automatic response to keep a lid on my horses nervous energy was to hang on valiantly determined to prevent him from accelerating. Opposing signals such as restricting a horses head before and over jumps or forcing a nervous horse to stand one minute longer beside the mounting block will make a horse feel claustrophobic. Restraining the jigging horse in a flatfooted walk is ineffective. Hold your horses is a useful idiom but less useful horse training advice.
4. Dont correct flight with punishment. Advice I received to slow down my
speedy horse was to promptly brake and back up on approach to the fence when he opted to accelerate. The same applied on the landing side if he charged away. I now know why it didnt work very well. Correcting a fear response with punishment is adding fear upon fear like adding fuel to a fire