Any horse experts or owners of misbehaved horses please help? :(?

i have a 5 year old mare, shes a section D named ella and shes a lovely well behaved horse! but as soon as im in the saddle shes so unpredictable, she'll run of with me on her at the littlest spooks, such as signs, car engines, birds, motorbikes, shes scared of alot of things, and today i was riding her down the bridal path and their was nothing atall to scare her, and she just stopped and i kicked her on to walk and she walked a tiny bit and sharply turned around and galloped of bucking at the same time, i pulled on the reins to stop her and she doesnt stop no matter how hard i pull! then she fell a little whilst i was trying to get her to slow down and i fell of and hit my head hard, i dont understand why she plays up for nothing?! its really getting me down because i want to try and train her for shows and theirs no way i can because ino she'll play up! im worried about my safety to :( please help what could be wrong with her?