Before I start really riding, I ride my horse in a small circle like 10-20 times. It gets the jitters out of both of us. I haven't had any terrible experiences, but the best thing to do is get right back on. When u ride, make sure you're not leaning forward on your horse...just make sure you're both comfortable!!
Breathe. When I was younger I never breathed while trotting and jumping, so I would always sing a song or something. But in this case, I would just take deep breathes, in through your nose, out threw your mouth, and focus on the simplest things (like Ella said) going in small circles, steering in in a small, slow walk, and just slowly ease into the bigger stuff once you think she's ready.