Go to mount, once she moves dismount and make her move. Make the good thing easy and the bad thing hard!
If it happens before you even put your foot in the stirrup stand there and wait for your horse to stop for 30 seconds. If it's when you put weight on the stirrup pull your foot out and back her up and do it over and over until she stands. If it's after you are on, mount up and make her stand.
Every time you get on and she moves back her up and stand for twenty seconds. With my mare I did this and it worked wonderfully. And after you horse gets it after maybe 20 or so times then she will always stand when you get on.
As soon as she steps forward ask her to back up and try mounting again. If she tries to walk off again stop her and back her up and continue to repeat