Your horse is behaved, has and shows manners and is working well for you...
The problem is theirs and their horse.
You should not be made to feel guilty for wanting to ride, work with your horse because the other boarders have a problem.
The fact you are not enjoying your horse is a problem for me...
You pay board same as them.
You have the same perks on the farm as they do...
So, I guess those that own the mare will not handle, ride or do anything with her out of sight of your horse...that is their choice and it is their problem.
What would happen if you decided to move?
They would be forced to deal with their horses nervous breakdown or get rid of her...again, not your problem.
Don't let their problem become yours.
Handle your horse, ride and do as you need to when you need to.
Don't subject your horse to working in less than ideal footing conditions trying to appease these people.
If you want to ride, do it.
I also would not
encourage a ride together if the mare is so mareish...and herd bound.
You could, probably will be held back in what you want, need to do with your horse in your limited amount of time for riding enjoyment...that is not fair.
I don't want to sound uncaring, but their problem is theirs, not yours.
Don't own it and don't make it yours either.