Reading this, I suggest contacting the seller if possible. They may be able to provide the training or direct you to someone who can, while you recover.
Drafts are different, they know their size & can be very pushy if not taught respect for the humans who work with them.
I say this as a neighbor has 3 Belgians - one 19H, the other two 18H+ - all with Show Hitch backgrounds. This is the use he intends for them. Ages are 9 to 13.
But all were working when bought.
The 19H was a Wheeler in a 10-horse pickaxe, as was another. All were driven single, team & in the larger hitches.
And all have ground manners firmly installed.
I have handled the biggest & he never put a hoof wrong, with all 5ā2" of me on the end of the lead. This when he had to go to a clinic & owner was busy with entry paperwork & consulting with the vet. Students also complimented his manners when we picked him up a week later.
Sunday I drove one of the others to a breaking cart.
Neighbor was in the cart with me, but I never felt like I needed to give him back the lines. W/T/Whoa all listened to. As well as standing while I got myself into the cart - that alone was no small feat!
That your mare was left untrained - due to size - IMO, does not speak highly of the seller.