I third the trainer suggestion. Find someone who can help get your mare manageable in multi-horse situations, and also help you learn to be more effective as a leader, worthy of respect.
Is your mare turned out with a herd? In my personal experience, turning a herd-bound horse out separately from his buddies has helped immensely. Even in adjacent paddocks at the same time, having a fence between them helped a lot.
As to the "buddy animal" idea, I personally wouldn't want to risk overattachment to any other "herd" than me, whether equine or otherwise.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown