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!!!
Please do not try to insult how I keep my animal.
Years of living out on a range taught many horses (and many still have that bred instinct) how to live that way, just like years of domestic breeding has inbred a sort of pampered intuition seen in a lot of well kept horses. Same with dogs, its why common house breeds are better for constant human company, and why some are better for working out in the field- its how we humans bred them to be.
Indie spent her first years in a herd, and was on the lower pecking order and constantly pushed around. She has lived on her own for the past few years, I tried introducing her into the herd life, but after nearly half a year she was so stressed at having to deal with another horse taking away her human time, and with this horse pushing at her and stressing her, she lost a considerable amount of weight (they were fed separate, so that was not the issue) and I had to find the foster horse a new home for Indie's care. So yes, I think it's safe to say she is quite happy and prefers to generally stay on her own. I've also seen other horses who just don't like being around other horses, they stay in their own spot, or happily wander into their stalls to get away from the commotion (most of them are mares from what I've seen moody little things.).
I was offering up another view point on horse keeping, there are many of them to go around, and like I said- as humans we have bred many different things into animals, and so there are different animals that prefer different things. And that is great! Because there are lots of people that prefer different things, and so there is always a match for someone.
Same advice I give for all horses, study the horse and figure out what they want and need.