Billions of years of innate genetics cannot be undone by pampering.....or breeding. Instinctually, a horse is a horse.
You can take a horse out of a herd, but you cannot take the "horse" out of the horse
If your indie gets stressed in a herd, it's because she's USED TO being isolated, which does not necessarily tranlate into PREFERRING being isolated.
It's odd that you haven't ever considered that maybe the reason being in a herd stresses indie is because she's never been allowed to learn to be a horse. And a horse is a herd animal.
Horses need a companions of their own kind in order to live as the Creator intended them to live....I wouldn't second guess the Creator!!!