Depending on the horse, they can be perfectly happy living with other species - it doesn't have to be a herd of horses for the horse to be content.
My mare lives a horseless life. She hasn't seen another horse in over 6 months and it'll be a good many more before she sees one again as I have no plans of obtaining another horse and we occupy the only pastures on the property we board on.
However, I did buy a pair of goats to be her friends and our pastures are the grazing areas for a herd of 5+ deer who are also Lacey's friends.
She seems to prefer the deer over the goats (as in, if the deer are grazing in one pasture and the goats are in another, Lacey is with the deer) but she's perfectly content with her goats.
And actually, I've found that when she's with other horses she seems to be more stressed than she is with her "not the norm" buddies. Of course, she is mostly blind = stress about the other horse's body language. I think though that if she can be really really happy like that, it's not a far stretch to think that a seeing horse could be, at the very least, content.
Pasture does not have to mean other horses. :)
And I am so sorry for the loss of your mare, That is a huge fear of mine.