I have a very respectful but indefinitely high strung mare. It's just her personality; you have to find ways to work with it and around it.
Desensitizing is great. It helps horses learn to deal with their emotions. Creates a controlled "life threatening event" and gives a horse confidence when they don't die.
I've found a couple of other things that have helped with my mare. It's good to have a bag of possible solutions with naturally nervous type horses. YMMV, but might be worth a try:
Firstly, I made the mistake of asking my horse to stand still when she's nervous. Sometimes, when a horse is having an emotional crisis, you have to work with them. My mare hates disobeying me. She knew she wasn't suppose to move, and that knowledge made her "claustrophobic". She couldn't get away if she felt like she needed to. This just made her more nervous until she exploded. When you start noticing your horse is getting nervous, lunge him in a walking circle around you. Sidepass. Back a couple steps,go forward a couple steps, back a couple, forward a couple. Whatever. Get those feet moving with PURPOSE.
Teach your horse to lower his head. Lowering the head naturally promotes calming in horses. Do it on the ground and under saddle (but I've found it more useful on the ground)
I'm going to sound crazy for this one. "Ground" yourself. Standing on the ground, imagine a flow of energy moving from your chest to your feet, dissipating into the ground and radiating outward like tree roots. If your in the saddle, imagine that energy going from your chest, though your horse's legs, and into the ground. Breathe. Look at the ground. Nutty, eh? But it works. Horses are very in tune with body language. I suspect that's why. They communicate very subtly among themselves.
Always keep the head free. Never restrain a scared horse by taking away the mobility of the head. On the ground, don't hold him back by the halter if he's trying to walk away. When you're under saddle and he's nervous, never use two reins at once. If a horse thinks they can move if they need to, they are way less nervous. It's like being in a room and knowing the door is unlocked. Even if you have no desire to leave, knowing that you COULD is comforting. Lock that door and all you'll want to do is leave.
I have tons more. If you think the above might help, I can write a more complete list. I come up with new stuff almost daily.
The life and times of being a the owner of a horse judges frequently refer to as "Nutter Butter".