Sounds to me like the 'trainer' just forced him to comply & returned to you a fearful, confused horse
Unfortunately not a rare thing, for people(me included) to find they have given a horse & lots of $$$ to the wrong trainer.
I would not 'trust' this horse either(except trust him 100% to be & act like a horse
), at least until you've shown him he can trust & respect you.
In that case, I'd definitely go back to the basics with him & get rid of that pain causing device, focussing instead on *teaching* him how to respond rather than forcing, & proving to him he doesn't have to worry about(& get bracy, reactive & unresponsive because of) you hurting him. Unfortunately my boy's experience taught him to be nervous of any headgear at all when I was aboard(he was fine on the ground), so I 'restarted' him naked(him not me!
I understand you don't have ideal controlled areas, but perhaps some electric tape(not connected) & treadins will give you cheap adequate security?
Yep, the goal is to move down the bit scale. ...I started my mare in a full cheek corkscrew, moved down to a full cheek
IMO that is completely backward & is only the case if you rely on coersion through pain - or the threat of. Each to their own, but I don't personally like or agree with that approach. I prefer to avoid causing a horse pain where possible & it's important to me that they enjoy playing my 'games' & learning.