Its self preservation and fear of the unknown.
This horse bucks once in a while. You fear for your self preservation.
You usually train your own horses and you haven't been active in this horse's training, which means you have no idea whats been going on. Fear of the unknown.
The only way to relax your anxiety is to get up there and get 'r done.
Scared, jittery, shaking hands, pounding heart. Just get down to work. Find your stride and your equine instincts will fall into place. You know what you're doing. You've trained before. Once you re-aquaint yourself with this horse, your fear of the unknown will dissipate, and once you start riding, your self preservation anxiety should calm by itself.
Set goals for yourself. "Today, I will do this", and then make sure it happens. Fear has no room to play when you're focused on reaching your goals.
Good luck and get us pictures :)