I have a gelding that is similar! Parelli-followers I know [ie, the bodyworker...haha] have mentioned to me that he is a very much a "Right-brain introvert," as well!
For the first year and a half I had him, he accepted me in his pasture but anyone else would cause him to head for the farthest corner of his 6 acre pasture, and, basically, hide until I showed up. If I was there, he would occasionally semi-engage with strangers but it was mostly very nippy sort of interaction [his go-to "I'm very uncomfortable" behavior]. If the interaction extended beyond his comfort zone and he felt like he couldn't leave, he'd retreat into his stall and put his head in the corner - essentially blocking everyone out. If he was approached while in his corner, by me or anyone else, he'd pin his ears, then snake his neck and then maybe snap his teeth, if he still felt threatened.
The only thing that has really helped him is time and not pushing him. He was out to pasture for almost 2 years with twice daily interaction with me [though often it'd be weeks without me physically handling him in anyway - I fed him twice daily and we interacted physically if it was necessary or if he wanted to, but it was very limited]. This management scheme wasn't really planned - I was [am] very busy and it was often dark [with no electricity at the "shed"] when I was home and able to do any work with him. So he sat and just bummed around the pasture with his two goat buddies.
After about a year and a half, he started "asking" me to work with him more often - galloping away when I asked him to come to his stall for dinner, then coming back if I ignored him, and galloping off again when I paid any attention to him, hovering around more as I worked with his goat friends, following me wherever I went, meeting me at the gate everyday - twice a day, that sort of thing. He started getting into petting-range with strangers and engaging with people he didn't know in "normal" ways [still reverting to nipping if he got nervous/overstimulated].
He also started doing some "boredom"-type behaviors - chewing the siding off his stall-shed, digging in his stall, all despite the fact that he had a grassy 6 acre pasture at his disposal.
I started trying to work with him more, but my schedule just did not allow for it too well, so I started looking for a boarding barn where I could visit after work, in the dark, and still be able to see him!
I found the perfect place, with people who treat him carefully...but not too carefully.
He has really blossomed since coming to the barn, but, without that initial "depressurization" period, I think he would be doing a lot worse right now.
He still is wary of strangers and doesn't hang out with his head over the door, but he watches everything that's going on in the barn and seems to enjoy the activity.
I see him everyday and that regularity is very valuable for him as I seem to be sort of a "safety blanket" for him.
At first, I was still riding him as I was under the impression that he was sort of broke, but, after moving the barn, I quickly realized that he had some MASSIVE holes in his training. So I stopped riding him, at all, about 6 months ago and we've just been doing liberty work and groundwork to increase our communication and build his confidence.
The liberty work has been HUGE for his confidence. He's learning to approach things he fears and that not everything plans to kill him!
Here's a video that I really love because you can see him processing A LOT of feelings.
The backstory for this is that I wanted him to trot over the ground pole, then over the little jump. We had been working a lot on little jumps and such, so he knew the individual pieces well, but he was overwhelmed by the two parts together.
As you can see, it took him a while, lots of tries, but he understood what I was asking for and wanted to do it. This sort of thing takes a strong relationship and time, but you can kind of see how I'm choosing when to tell him things and when I'm choosing to let him think it out. For him, letting him think it out is huge. It's a major confidence boost for him when he is right, and he seems to learn faster from his mistakes.
Right now we've been working through a lot of his baggage around someone getting on his back/the mounting block. He gets to rest and get lots of scratches, occasionally treats if he tries REALLY hard, by the mounting block and he has to work if he intentionally moves away from the mounting block without permission - all while I'm standing on the block, above his head.
It's been tricky, but we've gone from him completely panicking over me just standing on the mounting block to him marching right up the second he sees me on top of the mounting block.
I haven't gotten on, at all, yet, but I'm hoping that part is coming soon. I plan to just sit on him at first - not asking for movement, just getting him ok and not anxious about a human on his back.
The one thing I feel pretty confident in saying about this type of horse is that recovery can take a LONG LONG time. It's really possible and they're super once they realize that humans are ok, but it isn't a fast road!
Best of luck. :)