My Horse Stumbles Constantly When We Go Riding. What Can I Do?

Emily says, " When I go ride, she will stumble with her front feet multiple times to the point where she often goes down on both front knees. This happens going downhill as well as on flat dirt trails and meadows (she is a mountain trail horse). I've almost gone over her nose a few times and it's jarring to both her and I as I try to help her stay up. My anxiety riding her has skyrocketed over the past few years because I'm always worried and with my guard up for it to happen. It sometimes ha...