It is likely at this point that you've coddled him over poles enough that you've taught him to balk before them. Not everything we train our horses to do is intentional. Now that you've taught him to balk, you need to un teach it by doing it right 10x more than you've done it wrong.
This is the reason why your horse still balked at the pole the first time even though your friend was working with him and not you. It doesn't matter who is working with him when you've trained him that balking the first time is an acceptable response. He's not scared of it, if he was then he wouldn't go over it perfectly fine EVERY
time except for the first time. So now you need to expect him to do it right - the first time.
My suggestion would be when you lunge him and ask him to go over the pole for the first time, the second he refuses, pop him on the butt with your whip. (I know I might get flamed for suggesting this, but really, in my opinion, it's the only way to unteach something that has been so heavily reinforced.) Same thing with mounting. My horse liked to do the exact same thing. I'd walk her up to the mounting block and she'd wiggle and move her butt out. Not because she was trying to avoid being mounted, but because she didn't know what I expected of her. As soon as I figured that out I walked her up to the mounting block, as soon as she shifted her body away from it, I took my crop and smacked her rump on the side that she was wiggling toward. I then made her turn a circle and come right back around to the mounting block and there was no wiggling.
I know you said he's a sensitive horse, but my feelings are, that even sensitive horses like knowing what's expected of them. They like guidelines and rules. They like to know where they are in the hierarchy of your relationship, it gives them structure and makes them feel more safe and "confident." You need to "lay down the law." Sure, you could try over several sessions to encourage him to go over it with treats or something. But then, he's going over the pole on HIS
terms, because he decided HE
wants to, not because you TOLD
This can lead to other problems. What if you're out on the trail and need to cross a log? You don't have several sessions to work with him on it or any treats to bribe him with. You need him to listen to you NOW
, not later. Ideally your horse should do most anything you ask without much hesitation and that work starts in the arena, and starts with you telling him what to do and him responding on YOUR
terms, not his.
You being more confident and giving him structure and rules will help him relax, become a little less "sensitive," I bet you.
Getting a coach is a good idea.