My boy is six, and has been under saddle for about a year. From spending his first 5 years in a pasture, he definitely questions everything and it was very hard to break him out of the immediate 'run for my life!!!' Phase when the wind blew the wrong way.
Whenever I notice my horse getting tense, I instantly start doing things to keep him busy. Side passing, gaiting to backing up back to gaiting, small circles, big circles, pivots, constant gait changes. I try to work him so hard he doesn't even notice that he is nervous.
When he does spook at something, which used to mean a bolt, I would instantly start working him HARD. Lots of circles, backing up, gaiting him fast through the area that scared him multiple times. His long bolt turned into a short bolt, into bolting a few steps, into taking one scared step, into a big jump straight up, and now a nice flinch.
If he does do his big flinch at something, I do make him walk past it (not up to it) multiple times, both ways, until he acts like its something he's seen his entire life. I'm sure there are tons of different methods for different horses from different backgrounds, but this has worked wonders for my spooky boy :) good luck!
"Putting pads on a gaited horse is like putting a mask on a beautiful woman."