Similar to what macabremikolaj has said what I have found that works for an impatient horse when mounting, is to do the following:
This works best with split reins, stand to the side of the horse as if you were going to mount. Make sure they are standing still, place one foot in the stirrup and pause. If she starts to walk off, quickly take your foot out of the stirrup, (draw attention to the fact they did something wrong) have the outside rein laying across the saddle horn and simply back her up two steps. Then repeat the process, teaching her that if she tries to move out there is an imaginary wall that she is going to have to back up to avoid. Do this until she learns to stand for the mount. Then you as a rider needs to make sure that once you are mounted to consciously wait 30 seconds before cueing your horse to ride out. If she becomes impatient and takes a step, reinforce what you did on the ground in the saddle, back her up two steps. I am not saying you do this but if everytime you get in the saddle you immediately ride out, your horse catches on and will anticipate this. Instead teach them the opposite, that they don't move until told through you. Hope that makes sense, I have yet to have this method not work for a horse. And am constantly surprised at how many horses have this issue, you are not alone.