Honestly, it will just take time. Have you worked with stressed horses before? Take some time, a lot of time really, and work in a closed in space like a stall. Go slow, pet them a lot, talk softly and calmly to them, and slowly work on putting it on. Even half way, then take it off, praise. Work a little more, stop and praise. Go slow, be peaceful and calm, work on touching their whole face, just make it a really peaceful experience. It takes time and they probably have a reason its do upsetting, so don't expect to put it on and go riding right away. Spend as many days as it takes just reworking the basics and getting them to trust you to do that.