I don’t have any experience with rearing, but I did start to sour my Morgan early on in his driving and fortunately caught it before it got out of hand.
I grossly underestimated the amount of time required to leg up a driving horse. Sitting back in the cart/carriage I didn’t get fatigued and I didn’t have physical contact with my horse to feel him being tired or breathing hard. I was new to driving, disconnected from my animal and not able yet to read more subtle signs of tiredness.
As our drives became longer, he became sourer and more resistant to leave the barn. The worse he got, the harder I drove him, thinking the wet saddle blanket theory. It was wrong of me. Though he wasn’t drenched in sweat by the time we came home, or stumbling or anything, it was still too much too soon for him. Finally one day, he didn’t want to be caught for work, which was very out of character for him.
I went the other direction for quite some time, driving much shorter distances, same direction and distance each time for a few outings so he learned exactly what was going to be expected of him, and included a lot of random halt breaks.
This worked well for us.
When he learned he wasn’t going to be pushed as hard anymore, the resistance to leaving melted away.
I worked hard from that point on to slowly and carefully condition him, it took over a year, driving several times a week.