How do i keep my horse from backing up?

do you get on from the ground or mounting block? I have never mounted that way could be she is trying to tell you that you are doing it wrong. I always stand at the left shoulder with my left shoulder to her (facing her rump) and holding the reins in the left hand use the right hand to put my left foot in the stirrup (if the horse is tall) then holding mane swing up and around on their back. You get more more momentum this way and pull less on the saddle. Don't know if this will help you but hope it does.