I hate having my horses buddy sour. I have no tolerance for it. My two year old filly, though many may disagree, has mostly been turned out by herself in a square pen or a small field after being weaned. It also helped with training her because I was her only "friend", and she didn't have a herd of other yearlings and two year olds to be alpha with.
Sooo, my suggestion is to turn her out by herself away from the horses each day. Or you can turn her out by herself ever three or four days, then allow her to graze with the others for a day.
Jessica Fox & Shez Kinda Impressive
2009 APHA World Top Five SPB Longe Line
2009 ICPHA High Point Yearling Longe Line.