Start slow. Put your foot int he stirrup but don't try to mount. Put your foot in and pull it out over and pver and over again. Let her get used to that. When she can stand quietly, start putting weight in the stirrup, but still don't try to mount. Do that over and over again. Heck, jump up and down next to her until she can stand quietly. Basically, she need to be de-sensitized to what you're doing and when you mount from the ground, it pulls on the saddle weird which can be scary to a green horse. I hope this makes sense. Its alot easier to do
that to explain
In general, if a horse isn't sure about something, I do it repeatedly until they realize that its nothing exciting and becomes second nature to them.