I went through a similar stage with my first horse. He never bucked and was trustworthy and predictable, but tended to be very nervous. His MO was to go super stiff, and jump around to look at whatever caught his eye. He had jumped out from under me a few times, leaving me on the ground. After a spook, he would get hyper and any speed between a full stop and a stiff, jiggy lope went out the window. I went through a couple of months where I felt more comfortable, esp. trail riding, on almost any other horse. So, I taught him how to do a one rein stop in our pasture (at the time, we didn't have an arena). He got pretty good at it, even from the canter. One day, I went on a trail ride and he spooked, just like before. I one-rein stopped him and flexed him. Instant behavior, instant confidence. I knew that now, I could stop his feet no matter what happened. I teach all horses I ride for any length of time some rudimentary one rein stop.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown