I feel your pain.
You have to gauge whether your BO is actually willing to work with you to find a solution or whether she is just over it and would breath more easily with you and your horse gone. If the BO wants to work with you to find a solution, you have some room to negotiate and you might be able to retrieve the situation.
You might consider at least temporarily adding a mare-specific calming supplement to your horse’s ration. At the very least, this might buy you a little time and demonstrate your willingness to try whatever might work. (I have found for my mare that Smartpak’s SmartMare Harmony has helped a lot. But it has some ingredients that aren’t for those who show, so YMMV).
If your BO is done with dealing with your mare, then you’ll have to move. How a BO feels about a horse can have some subtle or not-so-subtle impacts on how they relate to the horse or react to it. I’m not talking about abuse or anything like that, but increasingly, for example, your mare may get blamed for anything that happens in the herd. Or, the mare may be left in for several hours a day just in case, or, or…
Good luck.