My horse used to bolt off and rear on me.. sooo scary. Until I started to joke about it. When something scared him I said (as we were galloping off..) "oh, it's the boogey man! Don't worry, I'll save you!" And by me relaxing and joking around, he began to relax.
It sounds really silly.. but the main point is when you are scared, your body tenses up, your lungs expand to get in more air so you can bolt off, and your heart rate triples! So, by making yourself calm.. making yourself snap out of the "OMG I AM GOING TO DIEEE" rut, things will be so much better.
Also, start from the ground. If your horse gets scared when you're brushing him (a flinch,) laugh it off and stroke his neck and say "oh my goodness, it's the boogey man.. you're okay though, I'm here! We'll kick his butt together!" Just anything to make yourself calm down.
Singing helps some people, talking to your horse, telling jokes, making every little thing they do into a "oh you funny boy" attitude..
Your horse acts up because it gets a reaction. Give it a calm aura to begin with and you'll be fine :)!!