I know you may think it’s dangerous but we’ve broke many horses of this habit safely by leaving them in “solitaire”. I would try it out of the cross ties first if possible. Let her cry! It’s like a child, you give her attention for doing that action she wins and will continue because she knows she’ll get her way. We typically start in a round pen or a stall far away so she can’t see other horses then gradually move to the cross ties. It really depends on how bad you want it to stop also. I hate making my horses suck it up but sometimes it’s what needs to happen. I’d also recommend maybe putting her on a ulcer med just to see if that helps with the nervousness. Personally I think it sounds like she just is bored and doesn’t want to be alone so I would just start with baby steps so you don’t get hurt. Stand in the cross ties holding a chain over her nose or lip to distract her from being scared. Everytime she goes to cry or move, giggle or apply light pressure on the chain. Just be gentle, stand where you could get away if it goes bad.