how I made my mares more brave is just ground work and being relaxed in the saddle, when I got my thoroughbred I was told she does spook and to be careful, first time I took her out to the trails I had her tacked up and I hand walked her out, along a train track (couple feet away) through a corn field, forest and if she spooked I would make her stand and just talk to her, be patient and kind to them. Don't get mad and yell at them that'll just make them more fearful. If your horse is mainly spooky lets say in a forest do lots of work there on the ground and get him/her comfortable being in that area and more confient. That's how I would do it.
When you see something that makes your horse tense up, distract him. Get him to focus on you, and not on the spooky object. Have him do circles, or a side pass, or something to get his mind off the spooky object. But whatever you do, you can get him closer to this object as you do it, and he won't even realize he's getting closer because he is focused on you.
Exposing her to lots of different things on the ground (bags, tarps, bridges, etc.) would be good for her. And when she does spook just take her right back to work and dont make a big deal of whatever she spooked at.
When she spooks, don't stop and let her look at what she spooked at. This just exaggerates the importance. Make her move forward and go back and forth past the object until she can go past it without looking or changing pace. You will have to keep and bend away from the object, but strong outside leg so that she doesn't shy away from it or turn.
Where she is spooky try and relax, flex her away from what she is spooking at so her body is moving towards and and repeat. Keep circling past it until she is relaxed. Repetition is so important with young horses.
You should try to get her back to work as soon as you realize she is spooking. Take her somewhere else in the arena and get her mind back on you by doing transitions, circles etc. Once you feel her attention on you, approach the spooky spot again riding confidently. If she shys away as you pass it by close your thigh with a lot of pressure on her side like youre holding her in place
Be confident, have contact on your reins so that she knows your there. Sit deep in your saddle, and when she spooks, try to point her at that object and ask her to stand so that she can look at whatever spooked her.
Do lots of ground work. Put a plastic bag on a stick, grab pool noodles, and everything that she spooks at. Rub it all over her body and reward every encounter with the spooky object.
I would do some ground work with her if she's very spooky. What I did with my mare is tied a plastic bag on a some kind of stick and waved it around while holding her on a lead. Rub it on her, go between the legs. You could try walking her over a tarp as well.
Remember to remain calm, not tense up and ride forward. Being confident and calm is extremely important when training green horses.
The main thing is not ahuytkng her down when she spooks. When you do this, your body becomes more stiff, and this sends the message to her that you may also be scared and that it is right for her to be afraid. You need to push her forward and just kee going like nothing happened. Don't hold back from sitting a little deeper and grabbing the mane is you have to. Afterwards don't be afraid to go back to the spooky place. Go back confidently but not pointing her directly at it. Then continue on with your ride with lots of circles and changes in pace so that she focuses more on you than anything scary in the future.
I would sit deep and try not to tighten on the reins (that creates tension which makes the horse more nervous) then make her think I'd something else. Trot, circles, side pass.