Ok I skipped alot honestly bc I didnt agree with alot, lol.. I dont like the egg or any kind of whacking on the head! Sorry! I'm not arguing I just dont like it. My gelding was a "rearer". I have heard of them working. I have even seen a horse go onto it's knees when a water balloon was smashed on her head. She stayed there for a few minutes.... I dont want my horse to think I'm hurting him. I want him to trust me. And I have also had a neighbor KILL his horse by flipping it over (this was almost 25yrs ago.). And that happened bc a horse falling on it's back ISNT graceful! He fell straight back and somehow broke his neck.
And it sounds like your having the EXACT problem I did! My boy was ALSO very concerned about leaving the herd! And this was while I was breaking him. He would try so hard to do what I asked that he'd overthink it, and after getting it wrong repeatedly, he'd get frustrated THEN would rear, THEN want to go back to his herd.
So what did I do? I started at groundwork away from the herd, and when he started flipping out and rearing to get back, I took him back to his herd! AND WORKED HIM THERE! Boy did he put up a fight.... for a few minutes... then he realized that going back to the herd DIDNT mean he was out of work. He had reared a few times doing it, but I stopped it from the ground BEFORE I got back on (mine was a flipper!). Then after he realized this I started again, and after a few minutes he GOT IT! And now it's been a year and no more flipping after that ONE day!
And by groundwork, at that time. He didnt get exactly what lunging was about. He would get so worked up and assume I wanted him to just run, and I had a hard time getting him to relax! So you may or may not have to do something different in the ways of groundwork. I'd start at the root of the rearing. And it sounds like you believe it's from being herd sour, so start at the herd!
GOOD LUCK! DONT WORRY! IT CAN BE DONE! I'm a novice horse trainer! My gelding was the second I have broke and I did it! So dont feel overwhelmed! :)
So glad your doesnt flip back! And just in case you havent make sure you know how to get off a horse that is getting ready to flip back. Just in case! Be safe! And best wishes!
Angels are people who can see and feel the pain in all creatures... -myself