GOODNESS, no one SAID you did things badly! Just that you should try a harness change, see if it helps horse backing off from weight!
This is why I suggested the tires, it helps horse learn to go into weight. If he handles a car tire fine, maybe move up to a big pickup truck tire, to accustom him to more drag and weight. In the photos he looks fair sized, so eventually moving on to dragging two tires is not impossible work for him.
In real life, especially with ridden horses, handling is designed to get horse moving AWAY from pressure. He is NOT allowed to push back when given leg pressure, lead rope on the halter pull, rein signals, backs up from a finger push in the stall!
In Driving we WANT him to move INTO pressure, which goes against all his conditioning. We have to TEACH him that he can manage pressure, weight on his breastcollar is NOT going to hurt or get him punished. New driving horses ARE often tenative at moving into pressure with a breastcollar, cart weight behind. They have to be taught to be “brave” and make the effort to go forward as asked. You the trainer, start with small loads, single person in the cart. Horse is SUCCESSFUL in moving you and cart, gets comfortable and then you add a bit more weight to the cart. Horse CAN FEEL the added (25 to 50 pounds) weight, now has to learn to manage “all the extra load” he feels. So you again work him with extra weight, go about using all your gaits, turns, halts, over a bit of time. He gets accustomed again, comfortable, and then you add weight again.
You make sure additions are small, he is NOT ALLOWED to fail at what you ask. So he learns to TRY when you ask him because you have never hurt him when you asked this extra effort. This all takes a while, as he puts on miles with the cart. Think how you would feel if you normally carried a 10lb. purse, and then one day you MUST carry a 100lb. purse! Big change, would take a lot of effort to manage with such a sudden change in amount to manage. Same for the new driving horse, has to learn to manage his load behind, be brave.
I SURE would not quit working with my new project of driving him! You ALREADY have had him hitched and going. So you back up and do tires, it is NO BIG DEAL. Review of his ground work basics, adding a step you didn’t know about is FINE. Horse seems quite sensible, has managed WELL with all the other stuff you asked of him. We do reviews if we don’t have enough time to hitch and drive, horse doesn’t care if it was beginner stuff, he can always be polished up a bit doing this.
Ours have done the tires, makes them think carriages are EASY to move! After the light cart like in your photos, they move on up to 4-wheelers that are weighty. STILL have to keep pulling what is attached behind.
It takes a long time to build a well-trained horse for driving, it doesn’t happen in 60 days or even in 6 months if you can’t work him often, KNOWING how to train a Driving horse.
Yours doesn’t sound “sour or ruined” because he is STILL cooperating with you and handler friends. He might be a bit confused at this stage, but not a throwaway for driving.