It’s very important to note that horses are like people, they need to interact with other horses. Not having interaction with other horses can cause your horse to act up, it’s good for your horse to get attached to others because it shows that they are comfortable and being accepted, horse herds are family and that bond is really special.
I’ve been in the same situation before, my horse didn’t have any inter-action with other horses ( she was abused and was photo be sent to slaughter) right now she’s a pasture horse with a herd... she absolutely loves it, yes it’s a little hard to get her away from the other horses to go exercise but soon she realizes that she can come back.
First of all you need to know that horses have a social instinct, this is why they live in herd, don't let your mare alone, she will be sad.
What you try is to put her on a field with 3 other horses or more so that she doesn't get too attached to one other horse.
You can put her out of the field for positive actions such as food, walking around on hand... Not only work so that you will have a better relation with her and she won't miss her compagnons.
You can also make her work around the horses (she will find it uncomfortable) and rest when she's away from them so that she will feel better with you than with the other horses.
I hope i could help you, good luck :)