I work at a barn with a few horses similar to those you're describing. I've taken it into my own hands to provide verbal encouragement to settle them down ("good boy! Gooood boooy" say it slowly while putting the halters on and when they walk well for you), keep eye contact, try to feel the way a horse feels and calm them through body language, pressure/release, and verbal encouragement. Works for me! Try to develop a relationship with each horse even if you only handle them for 5 minutes a day, make them remember you, and your calm, gentle, but firm nature.