Easy to get her over this.
Work with another horse in an arena, at the walk, you keeping to the rail, have the other horse walk towards you, keeping several feet to the inside of your mare. Be ready for her to want to go in the same direction as the other horse and keep her to the rail. When she is happy with this, close the distance. You can also have her stand still in the centre of the arena and have a horse work around her. Start with distance until they are coming closer.
Progress from the walk to trot and then canter.