I have had a few horses that have done the same. I first always check to make sure there are no pain issues that are causing the problem : back pain, sore muscles, joints, hoof problems, mouth/teeth problems, eyes.
My sister had a young aqha gelding that became so unflexible at the left turn, she gave asking him for simple left turns and would turn him in a full circle to the right just to get where she was going. I thought this rather amusing but helped out of course
Short lunging episodes to the left, praising when the he is willing to move left, small exersizes under saddle all helped quite alot.
One way to get the corners of his mouth "soft" is to put him in a small enclosed area such as a round-pen (maybe a large stall but personally I prefer a bit bigger area), bit him up in a simple snaffle, a saddle, or a sircingle.
With a lightweight rein, clip one end to the bit, and the other to a side ring on either the saddle /sircingle with just enough slack that he only tips his nose just a bit to the side. Unless it's a typical explosive TB, they will generally play with the slack, verses the tighness when they try to straighten their nose and will quickly teach themselves the pressure/release lesson on the mouth. I don't generally leave them like this for longer than 5-10 minutes and then do it again on the other side. It works wonders without you having to sweat.

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Hope your horse is doing better!