I too was very afraid of tying a horse out - in fact just recently a friend of mine told me about her horse, who a trainer tied out. She was tied very securely, but it took 3 days before she stopped fighting the rope. When the owner took her home she was very good, but was having trouble eating. She began choking on everything. Soon she couldn't eat anything without serious choke. Finally the vet put her down because she couldn't eat anymore. On examination it was discovered that her throat and neck muscles were so damaged by the pulling that made her unable to swallow correctly.
I had an issue with my two horses being herd bound too when I first moved them in together, away from every other horse.
Here's what I did and still haven't hadn't a relapse in over a year.
I took my smaller and and quieter, easier to handle of the two horses out of the paddock. I started walking him around the outside perimeter of the paddock - at first the other one was all a-panic following us around the edge. After the second lap the inside one began to lag behind and just watch as we circled the paddock. When she no longer watched or fussed I put the little one away.
A few hours later I repeated this. This time it wasn't half a lap before the inside one stayed relaxed ad didn't watch us as we circled. So I added another paddock into our circle, so we got further away but still in hearing and seeing distance. Again, at first she jumped and fussed and when we got back she was all a fuss, but the second time she didn't even notice. I put the little one away, this was their reward for being good.
They both realized "I go away, but I always come back". Once this was no big deal I added my house onto our "route" where for just about 1 minute we could not be seen or heard. But before they even noticed we were back. Again, at first the inside one came running over when we reappeared on the other side of the house, but the second time she just looked up. The third time she didn't look.
The fourth time my neighbor stopped by to talk while I was behind the house with the little one. I let him graze as I talked, he liked that, it was almost 20 minutes before I went back and the inside one barely lifted her head at our return. I put him away. The next day my friend was over - so I took the little one on our usual lap, but this time, behind the house I left for a walk in the woods. I came back about an hour later, my husband was on call just in case my mare lost it, but she didn't. We got a little nicker when we returned, but it wasn't worth the energy of trotting over to see us.
They had learned that we get separated, but they always go back together in the end. I did it slow and never let them get too worked up.
When this was solid I switched for my more reactive horse but didn't have any trouble there.
I haven't had any relapses even though it's not often I take one out. I got a 3rd horse too and no one seems to mind if one or two are taken away.