I have come to the view that horses have to face their fears throughout their lives. So much depends on how they grew up.
The most stable of the nine horses which I have been close to, was brought up in a school. He was a union shop steward by mentality and he refused to ever put himself at risk. He was a dull but very safe ride. I bought him to teach my brother. He'd been with a young woman and had taught her to ride. She had bought him from a riding school. I think he tolerated humans because he knew he had to. I never really developed an affection for him, but I did trust him. Later I sold him to a friend whom he taught to ride.
My cob Joe had a similar cussed streak in him. He'd spent his life in a trekking centre until they let him go. If he didn't like the rider, then he'd go down on his knees and threaten to roll on them. He never did roll, but the incompetent rider had got the message.
With hindsight, he fought with me partly because I put him with a classical riding instructor who tried to put him 'on the bit'. When, after a few sessions with her, he started to rear I told her to go away - but he never forgave me. He was pretty bombproof in so many ways. Little actually frightened him.
But then he discovered his ultimate weapon against all humans:- he would stop, balk, then whirl through 180 degrees to the left and take off at the gallop. When he discovered how to to it downhill, on a hard surface I had to give in. He was virtually lawless.
I sent him back to the woman who had known him all his life to re educate him - she was a professional rider (not a trainer). One day I watched her riding him, he respected her and he obeyed her without resistance. As a rider she had no style: she slouched, she smoked, she rode with loose, sloppy reins. One day I watched her ride him off into a looming hail storm. Later I watched him with her 4 year old son - whom he meekly took around the paddock sitting on his back. She used to ride one hore and lead her young son, sat on Joe.
Joe had formed a bond with that woman and there was no way by which I could replace her in his mind. Through a combination of circumstances I gave him back to her. He was, I thought, safe with her.
Six months later, he went lame again from a weakened check ligament and partly for financial reasons, she put him down. I don't know the details, I suspect she had let some stranger ride him
He had fought me almost to the extent of being lawless, yet he had worshipped her, who killed him.
You'll not make a bomb proof horse, it is either in them to be one or it isn't.
The rider can only bring the trait out.