I have had similar problems with my Arabian when I first brought him home. He was used as endurance horse, and on our first trai ride, oh boy… he was a drunk sailor, making his own collective canter hopping all over the place, and sweated up from nose to flanks! He was about 8 at the time.
What I did was taught him how to relax by starting on the ground. I would place my hadn on his pole and apply pressure. When he started moving his head down I released the pressure and rewarded him. I used clicker training for this, but (noto many people like using food as a reward) you can reward in any fashion. Once I taught him that I wanted him to put his head all the way down and keep it there, I transitioned it to a halter and lead rope then after that to the bridal and reins. On my training rides in and out of arena I used the method, but I would not expect too much so that we both weren’t dissapointed. Slowly expectations raised. He had a high head trouble though, so this helped with that and calming him down.
Clicker method helped too because he was “clicked” and reinforced when being calm and walking not when dancing around like a lion would eat him. We just kept riding around the property and the roads until I could get on him without any prep work besides stretching. The clicker is good to capture a behavior because horses are so reliant on sound. It later helped when he became spooked at different objects.
Also, if you don’t think the above will help you, you should assess what troubles the horse is having and narrow it down. My horse had trouble with me riding him through a gait without taking off down the road. So I worked him like no tomorrow to get him to be calm. If he couldn’t go through walking he couldn’t go at all.
I also, if he would blow through everything I taught him, would circle him using the “Emergancy Stop” rein pull. I would be very gentle and light handed circling him until we came to a nice, quiet stop. Releasing the rein we would do a small circle around once and continue on. This gave him a chance to come back to working with me instead or reacting and getting hot. Redirecting his thoughts and energy.
There came a point that I had to teach him to be quiet in his “whoas” and stand still until I said “walk”. This proved troublesome when he wanted to go…
Another thing to consider, though you said your horse just had dental, is to consider ulcers along the gums. This was found in my horse’s mouth when he arrived home after the sale. Though your vet would have seen.
Consider too, how much are you feeding your horse in sugar? NOT treats necessarily but the sugar content of all the horse consumes. Is it heavy hay with alfalfa? Or too much grain? maybe sweetfeed? I heard this from my ferrier once… “Too much sugar, not enough activity, equals one hot horse.” Which is true. Though you may believe you are working your horse enough, are you really expelling all that pent up energy?
Hope this helps in anyway giving a new idea to you. Let me know what you end up doing. I love hearing new training methods!