An in stall mirror can help so they can see another horse if your other horse isn’t around. I know they make special mirror to go in stalls that don’t break
Consistency is the best thing for it, the more you take them and make leaving an enjoyable experience the better they will get. You just have to remember to make their jobs enjoyable. Give them some grain when you bring them into the arena, find their favorite scratching spot, don't drill and exercise for too long just make it short and sweet. Pretty soon they will start looking forward to their time with you
I used to have the same problem with my two. You can see my arena from the stalls. So i just took him out on the lunge line everyday and went a little farther away from the barn each day.