Lime Kiss
This video might interest you: http://www.parellinaturalhorsetraining.com/video/herd-bound-horse/
We board our mare with a little arab who is very attached to our mare. If she gets left behind when we do work with our horse, she screams, she works herself up into a lather and she has nearly made herself colic a few times. We found it helps to have a person that they are bonded to hang out with horses that are kept separately, so that they're not feeling vulnerable.
If she's the lead mare, she might just be worried about her herd and feeling that she has to protect them.
I'm sure you can find more ideas if you google 'herd bound horse'.
Good luck.
She's 2, just shy of being 3, so she's not the lead mare the other horses boss her around every chance they get (she's not the bottom of the herd though she's a feisty little girl) and even when I'm with her she's constantly looking for them, or if I walk away for her she starts looking for me me. I put her in the cross-ties while I went to get her stuff to lunge her and by the time I came out of the tack-room she was rearing in the cross-ties. Something she has NEVER done before. She is usually extremely well mannered.
I've had her for quite a while and she is well aware that I'M the boss and when she's around me that I'm the "lead mare" for her. Today I had a good friend of mine, who she sees EVERY single day and who she has bonded with, hold her while I put the other horses out and even then Bella did not give a hoot about the person it was all about the other horses.
She's currently being turned out with one other mare, who is fine without Bella in the pasture, and then four geldings. None of which care if she's in the pasture or out of it.
I got to talk to my trainer today who said that it's pretty much her just being young. She didn't have any advice on how I could work with her to get her over that yet though. Today training was nearly impossible. She was doing what I asked but her attention was clearly not focused on me.
I will check out the video, but if anyone has any suggestions or knowledge for me please, I'll at least read/listen to whatever.