My horse, also an Amish horse was 200 yds down the road before I sat down in the cart.
You are going to have to start almost at the beginning depending on how bad it is.
What I would do is:
Put your foot in the Stirrup. Start to mount, Don't swing your leg over, then get back down. Do this until he doesn't move.
Next, do above, but swing your leg over and sit in the saddle, Then get off. Do this until he is not moving at all during this process. If he even feels like he wants to move off. Don't advance to the next step. Continue waiting longer and longer until you get off.
Next step. Ask horse to walk a step or two, wait some. Then get off. You might enlist someone to hold the horse at the beginning of this step.
Next start walking and waiting. Walking and waiting.
With my horse it took about a month. Talk about a pain, harness and hitch up just to stand for ten minutes or walk for ten feet just to unharness but it works. Every time, I still will wait at least a minuet after I get in the cart before I ask him to walk on.
The best time to plant a tree was 20 years ago, the second best time is today.