Following on from the discussion about dietary supplements in the Horse Care section, I’d have to say: it depends on what is causing the behavior that you want to change. If the horse has a deficiency in a specific mineral such as magnesium that is known to cause anxiety problems, then feeding that mineral might help. If the horse does not have a deficiency, it probably won’t help.
In general, spooky and anxious are problems associated with training and management. “Hot” is more a description of energy level.
No one wants a spooky or anxious horse.
Some people want a hot horse, and some don’t. Hot per se is not a bad quality; it is necessary for high performance and speed sports. But a given hot horse might be too much horse for a particular rider.
The obvious time tested ways of handling excess energy is diet (less grain), turnout, and more exercise.
I think that until you have made all those changes in lifestyle, and done some confidence-building in-hand work for the spookiness, it’s a bit pointless to be looking for a magic bullet in a supplement, unless you are going to outright tranquilize the horse every time you ride. That of course works, but it isn’t show legal.