My Percheron was the same way. Deathly afraid of everything and knew how big he was.
He drug me across the parking lot more than once until I figured out to let go of the leadrope.
That said, a lot of good solid ground work will be the key to your success. I wouldn't even focus so much on riding right now. Start over, back on the ground.
Lots of repetition but not for long stretches of time. You need to keep their mind engaged.
I used to set up a lot of obstacles in the arena. Anything I could find and worked with Solon going around/over them. One by one we worked through them.
As far as respect, a good working knowledge of how to safely use a stud chain will get that in check. If you don't know how to use a stud chain, find someone to show you. It does wonders when used correctly.
Drafts trots are not bad. It's a matter of practicing to get smooth gaits.
But again, I wouldn't focus on riding as much as I would ground work. If you don't have a good solid foundation with a draft horse, they will know they can walk all over you.