If you hear a scary thing coming up behind you, you can turn her to face it. It helps my horse to have a chance to get a look at the monster. The more you ride, the more your horse will get used to new things.
I had a MFT that had a HUGE problem with things coming up behind him. You couldn't pony other horses off of him, drag logs or allow bicycles etc. (especially bicycles) to get behind him. We got him over it by finding the longest distance that he would allow something to be behind him before he would spook...just mildly. We then (using my son and his bicycle) brought the bike about three feet closer than that mark, allowing him to see him there. We then began walking him in hand, keeping the bike the same distance away, as soon as he gave a sign of relaxing (deep breath, licking lips etc.) we stopped the bike and allowed him to walk away from it.
We would then bring the bike back in, three feet closer this time and did the same thing over again. Once he was good enough that he could follow him with the bike just a few feet behind him we did the same thing mounted. Mounted it went really quick since we had already done it on the ground. Once comfortable there, we then parked the horse in the front yard with me on him and had my son ride his bike from behind the house into the yard (about 50 feet away from him) and started to ride circles around the horse. The first time was a rodeo and it scared him to see it (the bike) come from nowhere and I learned 2 valuable lessons. First, I should have done it in hand and secondly I was creating some of the spook as I was tenseing and grabbing the saddle horn just a bit to be ready before my son brought the bike out. I found this out by...my wife walked out to tell me about a phone call she had and my son not knowing this came barrelling around the side of the house (too close to the horse...he didn't mean to) and all Pepper (the horse) did was jerk his head up and snort. No jumping or spinning, nothing. I had been totally relaxed talking to my wife and wasn't ready for the bike to come.
So, sometimes I think we may "spook" before the horse does, causing them to spook. Sometime it doesn't feel like it and what we may feel is "prepared" or "ready" to the horse could feel like tension and the precursor to "run for your life". The biggest secret I have found is to find a starting place (thanks Clinton Anderson) and then push just past it allowing the horse to become relaxed or shows signs of relaxation and then take the stimulus away. Bring it in again (maybe a little closer or stronger) and repeat. Over time you can overcome nearly every obstacle if you are patient enough, have good timing and find a starting place. One of the best descriptions I have heard of how to solve horse training problems is to (I am paraphrasing) "put the horse in a bind (mentally or physically) and allow them to search for the right answer. Once they answer the question correctly, it is our responsibility to tell them (release the pressure / take away the stimulus) so they know they answered correctly."
If you are able to time this effectively, it is amazing how fast you can get a horse over a problem. If you release slowly or don't recognize the horse trying to find the answer, the problem can go on for a long time, never get fixed or even get worse. Timing is everything and I really believe it is the secret to the whole training process. You can teach proceses but you can't teach timing and feel and being able to recognize when the horse is answering the question correctly. To me that is the fun of horse training. Good luck with your issue and have fun getting her through it.
I had an arab that blew up one day when a motorcycle came by, So into the arena we went, I had a friend get on his atv and me on the horse and we chased the ATV around the arena. The horse was a lot less spooked of it chasing the ATV vs the ATV chasing the horse.
After a while, the atv got behind the horse and we worked on that position for a few laps. Then we started criss crossing each other. ATV doing laps around the arena and the horses cutting thru the middle to catch the faster moving atv.
After about an hour of playing chase. The horse could care less about ATV's. I give him a refresher when ever I drag the corral with the harrow. But it's no longer a problem.