Let me start by saying that you cannot get much training done on a herd-bound horse until you 'fix' this dependence. Horses have two distinct 'modes' of being. They can be in a 'responsive' mode. This means they are able to listen and focus on you as you interact with them. OR --- They can be in a 'reactive' mode. This means that you will have little or none of his attention and little if any cooperation.
There is more than one way to get a reactive horse to turn back into a responsive one, but, some ways are more effective and better than others in my book:
1) You can beat or bully one until you have its attention, but it is still going to be reactive, at the very least, to you. Not a very good way.
2) You can lunge it or round pen it to the point where it is too tired to react to whatever set it off the first time. Still not a very good way in my book.
3) You can tie the reactive horse up in a safe place until it, the horse, decides to get over it. Like everything else you are smart enough to let a horse teach himself to do, it is by far the most effective. For one thing, it takes little of your time. For another, it actually 'teaches' the horse something positive that carries over into subsequent training / riding sessions. This is something that longeing or round-penning does not do.
Longeing and exercising a horse before riding just make a horse tougher and more full of energy expecting an exercise session before each ride. For me, a 'broke' or trained horse is one that you take to the tack room, it stands respectfully and quietly while you saddle and bridle it and stands quietly while you step on it and ride off. A horse that requires anything more than this to prepare it to ride simply has a HUGE hole in its training. It has literally trained the rider to go through all of these steps that are unnecessary to do with a well trained horse.
I think this particular mare is reactive and so herd-bound she is dangerous and completely out of control. I think she has just gotten spoiled in this process. I have seen many just like her. She wants her 'pasture life' and has learned what she can do that makes a person 'back away' from her and put her away. Remember: "He who moves his feet first --- loses!" She is winning EVERY round here.
Is there a safe place you can tie this horse out away from her friends and everything else? This is the most effective way I know of to handle a herd-bound spoiled horse. I just tie them up until they relax and get over it. I have had it take an hour and I have had it take 3 days.
First of all, you need a good, safe place to tie. I prefer a nylon rope with a big SWIVEL bull snap. I have one hanging down from a large tree limb. It should be about as high off the ground as the horse's withers. IT ABSOLUTELY MUST HAVE A GOOD SWIVEL IN IT SO THE HORSE CANNOT TWIST UP THE ROPE AS IT GOES AROUND AND AROUND! If you are in the north where it is cold and covered up with ice and snow, just wait until warmer weather. This horse could get really hot and sweaty and it would be wise to wait for better weather.
I will tie a horse out early in the morning after they have been fed and watered. Then, I just ignore them. If they are still screaming and fussing at noon, I will offer them a drink (most won't take one) and let them stand until evening if they don't settle down. If they are still fussing, I put them up and feed them after they are cool and watered out. The next morning I do the same thing. Like I said, I have had it take three days on some really herd-bound horses.
When a horse finally settles down (and they will ALL finally settle down), I put them up. The next day I tie them out again. Usually they only fuss for a little while on subsequent days. I do not interact with a horse at all while it is learning separation and patience. I offer them a drink at noon and that is all I do with them until I put them up for the night. It may take a while, but, the result is worth it.
This teaches a horse to be comfortable with themselves. It teaches them not to depend on another horse. It teaches them that there is life after separation and they will see their friends again. It allows them to be in a 'responsive mode' so they can be schooled and learning from the moment you halter them and bring them in. The horse that has learned to live comfortably with itself will be more settled and quieter than any horse that is 'worked down'. They learn more and better than any horse that is in a state of anxiety.
One always has to remember that 'horses are creatures of habit'. Whatever routine you do repeatedly with them is what they will expect each and every time you work with them. If you are constantly exercising them to gain control and the upper hand, they will just get more fit and require more and more exercise. You will never turn this horse into a well trained one that can just be saddled and ridden. I am getting old and pretty crippled up with arthritis. I don't have the ability any more to work a horse before I work a horse.