I agree with everyone else, go back to the beginning.
I purchased my boy, Pilgrim from the Amish. When I first drove him I would step into the cart and he was 100 yards down the road before I sat down. For the first 3 months all we did was walk, stop and stand. Some days we would just drive 100 yards and put him away. Some days I just got in the cart, stood for 10 min then put him away. He did not know there was an option to go at any pace besides a, fast as you can, trot. I won't lie to you we still sometimes struggle with it. He is 1000% better but he still gets strong. Transitions in the trot helped a lot, going from slow trot to fast trot then back to slow, he never knew there was such a thing as a slow trot. He really likes it now. Bending when he got strong also helped. But this is all later work for your pony, you need more ground driving first.
Even though you are driving your pony, she is not trained to drive, what you have is a runaway, which you know is dangerous.
Another thing, sometimes if a horse is pulling too much weight, they can become a freight train when they pull, or they will balk and refuse to pull. Check out the weight of your buggy, you don't want it loaded to be more than the ponies weight if you are driving on good solid level ground, less than that if you are driving on rough or muddy terrain.
Good luck, keep us posted. You will have a nice driving pony when you are finished, good for you to recognize the problem and start to fix it. I know the ground work can be dull but you will have a safe pony when you are finished.
The best time to plant a tree was 20 years ago, the second best time is today.