Just don't finish getting on till she stands. Every time you start to mount and she moves, drop back to the ground and make her start over.
Position her up, collect a rein, maybe even slightly tip her head toward you. As you start to mount, if she starts to move off, pull her head on around and either stand in the stirrup ( don't throw a leg over) or drop back down.
They learn real fast that if they stand still, you will finish mounting. If they move, you repeat.
Once in the saddle, Don't GO. Sit for a minute fiddle with your tack, put on some gloves, drink a bottle of water. Every time she tries to walk off. Lift the reins, I'd even back her up to where you started. Don't allow any forward motion until you ask her to go forward. Your horse should understand a Go Forward signal. And she should ignore any other movement of you messing with tack or getting things settled.
I used to do NATRC competitive trail rides and judges would ding me points because my horses would move as I mounted. Once I decided I didn't want to loose any more points and spent 15 minutes a couple days in a row. Problem was solved.