I think everyone had very good points for you to consider. Start by eliminating physical probelms. I personally don’t care for cross ties as they can be dangerous when a horse is acting like yours is. They can flip themselves over and break their neck/back. Use a rope halter to ground tie your horse using an attached lead line. Have a few treats in your pocket. Ask her to stand still and not move. When she moves give a downward tug, not hard, and tell her whoa. As soon as she stands quietly or even gives a good try give her a treat. Tell her good girl. My mare when I bought her didn’t know how to do anything. I started her with a rope halter and taught her the word whoa. When she moved I gave a downward tug on her halter. When she stood still and focused on me she received a treat. She caught on very quickly. She now ground ties and stands for long periods of time no matter where we are.