How to gain horse's complete trust....?

I have an 8y/o, BEAUTIFUL red dun QH who rides great, is the sweetest thing on the ground, hardly any vices.... the only problem is, even though she is triple registered, she was mistreated by the owners before the ones I got her off of. Now, she is wary of me and hates the cross-ties. She'll stand perfectly still for you to mess with her if she's OFF the ties, but if she's on them, she fidgets and tosses her head to no end, and won't let you go near her head. Unhook her, and she's fine. I know she's always a little nervous on the ground, no matter how tolerant she is and how much she lets me touch her, etc. She doens't resist handling EXCEPT for when she's on the cross-ties. What can I do to get Cleo to FULLY trust me and trust the cross-ties? (Mainly me; I can live without the cross-ties, I just don't want my baby to be scared.)