OP- it sounds like you have a more complicated horse, psychologically, than you bargained for, but it also sounds like all of these issues may be part of this particular package and that there are solutions. Hopefully you can figure each other out and go on to build trust and a good partnership.
My two current horses are both imported. One was 11 years ago, the other was 2. Both are WBs with very little blood- you have to go back several generations to find any TB, which is a blessing and a curse. The blessing part is that they are usually easy to ride down- as McGurk points out (many/most?) TBs have no bottom. The curse is that they donāt have that lovely TB brain that wants to connect and they both think their stamina is about 50% of what it actually is- while a TB thinks their stamina is 500% more than it is. Communication is a bit like clear water (TB) versus somewhat more of a gel (at least for my two WBs, I donāt want to generalize too much, since there is so much variation across that collection of breeds and types!).
None of this is to say my guys arenāt perfect for me- I adore them both, but it took me a while to figure out the first one, and even longer for the new one. The first one survived his crossing pretty well- he arrived in good shape, and was able to settle in and sort out his feelings pretty quickly. He was (and is) very businesslike and had not much personality when I got him, as he had been an ungelded young horse in a very busy sales program, not a spoiled ammy horse. He quickly figured out that being an ammy horse is the bomb, but retains his businesslike approach to stuff. If I ride him the way I need to (balanced, forward, working over his back) heāll jump a house- any distance, any situation. If I make his job confusing or too hard heāll stop. Simple (except the fact that Iām an ammy and sometimes do dumb ammy stuff, but this boy has made me a MUCH better rider).
My second one came off the trailer looking like heād been beaten and starved (neither of which was the case!). He was so scared and worried and wound into a little ball it was shocking. It took me (and my trainer) quite a bit longer to figure him out, and to work through the issues. A big one was the TINY amount of ulcers he developed, probably from his trip- from his over-the-top ulcery behavior everyone thought the scope would show a riddled gut. No. Two teensy, tiny little areas were enough to make him feel like he was going to die.
Poor little guy! We treated that and things improved, but he was still pretty scary to jump- heās very, very, VERY x 100 spooky and he was used to being ridden by a big pro who would lift the horseās head up, get him moving forward, and then just very matter-of-factly go to the jumps. He goes from that to little ammy me, sort of floating the reins so horse is on the forehand and his extremely powerful hind end is rocket launching us over- and then he would often do a huge buck on the back side. Or, he is spooking so hard we donāt actually get over the jump in the first place. Iām a fairly good rider and very experienced with difficult horses, but this one took us several months to figure out. He didnāt get me off more than a couple of times, but one of those was after he leaped a pole like he was fleeing a fire.
Like my first boy, this one is not actually difficult, it turns out, he just needed to be ridden very differently AND he needed to build trust in me. But while a good TB will seek out that bond and trust, and then try their heart out, my two are a bit more passive about it- if I ride like crap they arenāt necessarily going to fill in my gaps. If I ride well they will do anything for me. All of this is to say that it takes time- sometimes very little and things click and you can meet each other on even ground and get to work, and sometimes months, which can be disheartening. I was not sure my second horse was a good match for me and was fully prepared to sell hs brilliant self if needed, but Iām so glad I decided to try to work through it. I love and appreciate him now, in all his spooky dingbatness, and have a lot of fun with him.
So, TLDR- stick with it and hopefully you can find the keys and enjoy your boy.