Sounds as though your TB is quite a handful. I understand and know how you feel. I have had horses like this and it makes riding -not much fun any more.
I was going to suggest you see if he was a 'rig', but you mentioned you have had him for a while and have not had issues until recently! I have found establishing a herd leader relationship with your horse helpful.
I like TruCharms idea of being herd boss.
And you also gave a good key when you said ...
"Take him into the outdoor arena and work him on the line until he's calmed down. Hop up and ride him a bit. As soon as we stop and leave
the arena he's a mess all over again."
A) he calms down when you work with him
B) he is rideable and focuses on you
C) when he has no leader he becomes leader again!
So - Be herd leader.
You can do this by lunging, working in a round pen, ground work with many of the horsemanship exercises available. You may find it helpful to enlist the assistance of a local 'cowboy' or knowledgeable horse person to give you some tips on how to proceed if it is a new concept for you.
Focus on being the boss and establishing respect within the 'herd' (namely you and your TB).
This may stem from a dominance thing or maybe he is insecure and requires a strong leader to give him confidence. USING HERBS/Essential oils
I use essential oils and direct pet and horse owners how to apply essential oils to help their pets, on many different levels (physical, physiological, psychological, emotional and 'spiritual'). I bring my expertise as an Certified Animal Aromatherapist
so horse owners understand the complex nature of essential oil therapy and how they can use these oils to calm/heal and emotionally assist their horses, and themselves.
As far as calming goes - the standby 'go to' oil is lavender
. It is known for its calming influence. There are many others oils you can use (geranium, vetiver - for grounding, but lavender is common and you can purchase it at most health food stores (just be careful of the quality) and it is safe to apply AND it smell lovely! :)
I have had success with lavender with an Arabian pony who was hot/cold. That is to say run away and lethargic all in the same riding session. I put one drop of lavender on my hand, rubbed them together and held them under his nose. He immediately pushed his muzzle into my hands and licked my hands. He was calm and interested in his work during his ride.
You may also try Marjoram - this help relieves anxiety, calming and an added benefit of helping reduce amorous advances of geldings (nudge nudge - wink - wink). Put a drop on your hands, rub them vigorously together to warm the oils - then offer the horse your hands to smell or lick.
If you would like more information please feel free to contact me.
~Laura at thistleridgestables.com