Start by figuring out what is triggering her to move off. Are you brushing her sides on accident? Start by raising your left foot towards the stirrup. The second she moves, back her up and make her stand. Tell her "stand". Try again. Every time she moves, back her up a few steps and tell her stand. Once she accomplishes standing for the foot, give it a rest and walk her around a bit as a reward for standing. Next actually step up. The second she starts moving forward, pull your foot out and repeat the backing. It sounds like a long process but if you go at it like you have all day and that you guys wont get started until she stands, she'll catch on pretty quick. Once you are finally able to swing a leg over, if she takes a step forward, get down and back her again. Keep doing this until its solid in her mind that *you* tell her when to move. Its not her choice, its your request. Get up and down a few times...your legs will be burning but it helps.
This is usually a horse anticipating that every time you get on, you go forward. Switch it up on her. Every time you get on, back her up. Or just make her stand for a full minute.
Hope this helps!