Step Horse Mounting

I just bought a horse and her previous owners didn't use the stirrups to mount they would pull them selves up with the Saddle horn so she learned that one there was weight in the saddle it was time to move. Im not the tallest person and she is 16.1HH. As soon as i put my weight into one stirrup she wants to move out, if someone is holding her for me she gets agitated and dances around and shows signs of bucking because she doesn't understand why there are holding her back when she was taught to move out with weight in the saddle. I've tried many methods like making her side step along the outside of the round pen, making her move backwards still she stands still to mount, lounging her still she stands still. How can i break her of this?

Also i like to give my horses incentive to move forward with pressure from my heels (NOT KICKING) and is scares her, she will start to buck and wont stop until im off her back. How can i break her of this?