There are different levels of separation anxiety. In this sense, only you can decide where to take your horse and how long each step.
Start with separate power supply. For each feeding I would remove another horse. In this way, they will begin to associate with leaving the bead something positive
Go back to basics. Start with the floor work. In the beginning, but they come out the closet, in sight of their buddy. Ask them to back up to stop and give them a pellet. Then they begin to walk out of sight of their buddies. Once you start your horse feel fear, ask for the attack, a few steps back and give him a tablet. You may need to repeat this several times .... Remember this is a process. I use treats for a horse to the very short term solution relink anxiety / fear to be something positive. When I stop a bad behavior something positive I can not believe it's a good thing. A frightened horse will not respect your space and I think it is important for a horse to learn, especially for your safety.
Each time you repeat this exercise a step further. Even when you see the fear, the stop back and ask (if necessary) to give a pellet. The key is to the horse something else to focus.
After repeated success with moving horse in the eyes of the dude, you can attend to further work on the ground. Enter your horse in a "job" to do an excellent way to him. Depending on your horse for training, this might work in a round pen, saddle and un-saddling, brushing, walking in circles, changing the pressure and the side is ...
As this happens, do not forget the buddy horse. I want the dude to give a little hay to distract him while I work the horse first. At first, I usually keep the horses that I corral in sight until the dude started removing food ... If the buddy horse starts walking the fence, he finds it tempting hay, and it will focus on something he likes.
Obviously, the term varies from your horse. The key is not on your horse. Once you have them a bit they'll take a mile. Firm but not hard.
Once your horse is comfortable with a certain separation, go driving, and again, focus on the essentials. Under saddle, what you did on the floor ... before the attack back to the other questions pass, go through doors ... etc. This will strengthen and build their confidence and comfort with you. Remember, your horse is a comfort zone, and the more you can expand that comfort zone that will be more successful and fun you have.
If you have a friend who you ride with (a quiet horse is better!) Can your horse (one time), to a new friend to introduce.
A good relationship with your horse based on trust. If this trust is established, I am sure you will succeed! After some time you ride your horse alone, or just run away from a group of horses at your command
Good luck to you and I will be glad about your progress!