I had something similar with my Morgan. The jog cart shafts were lively and wiggly as compared to pvc and saplings being drug and he got very goosey.
The first thing I would do is, just in a halter and lead rope, pretend to put him to but not actually as he’s got no harness on. I’d hold up the shaft of my jog cart and then, standing at his shoulder, ask him to walk on… shafts around him, me holding it up as we walked. He’d feel the wiggly bump and shoot forward out of the shafts, circle around me (me holding onto the lead rope, still walking calmy with the jog cart). I’d just keep walking, do our “lap” that I’d set out as a repetitive route. Come to the end and “re hitch” and do it again. It didn’t take more than a few minutes for him to realize that he was over reacting and didn’t feel the need to burst out of the shafts. The same repetitive route we took really helped a lot too, we just wore a path in the dirt taking the same oval over and over and over, stopping at the same place to hitch/unhitch/reward. I used anticipation to my advantage for this.
I’m a believer in if you let a horse leave they will learn on their own to stay.
Then when ready to really drive with harness and blinkers, what I would do is pat him a lot quite strongly and rub him good and firm before and as I hitched him, and keep having my helper pat and rub him and loudly tell him how good he is as I hitched. Sort of be louder with praise and good things than the wiggly shafts are goosey and spooky.
Then we’d walk off, me driving, friend or friends clipped in (halter over bridle, long lines with friends) and they’d keep patting him strongly and loudly telling him what a good boy he was. Etc.
Now, take this advice with a huge grain of salt, I have likely desensitized my horse too much. I pat my horse a lot, all over the place, while he’s hitched. I’ll bump him with pails, let bags rattle on his legs, etc. Its nice when we’re in the woods, we can go through a bit of brush, snap twigs, have debris fall on us, etc., and he’s rock solid. However, he’s learned to become quite lazy and the whip isn’t the motivator it once was either :lol:
I took a driving class once with a really wonderful woman, and her amazing horse. I went to head her horse and patted him on the neck before I got to his face (as I do with my horse) and he really jumped. I was shocked and felt terrible. She told me to never pat a horse in harness… which I didn’t know. Her horse was very sensitive and very quick to comply, I can only dream my horse could ever be so quick and clever to drive.
In making my horse solid and able to fill in for my greeness, I’m pretty sure I dumbed down my horse, but on the flipside, we’ve survived me learning so far :lol:
Early in my life with horses, I spent a summer with a John Lyons trainer so my roots tend to be with desensitizing, breaking things down into very small parts, and conditioning through repetition. This comes back to haunt me sometimes as I tend to over desensitize which robs me of lightness and forward when I’m finally ready for it.
From what you mention, I’ll agree with Munching, I think your mare just needs some time and support and she’ll get past this.