Try this on for size... A horse needs to be taught that it is not trapped in being isolated when separated from other horses. I train new horses the following way, but old horses with bad habits can be more difficult. Ride the horse away from the 'barn' or other horses. After a very short distance when you start to feel the horse get slightly antsy, walk back to the barn. When you feel him relax by dropping the head, licking the lips, cocking the leg, etc, go out again. Repeat this many times while going out a little farther and then coming back not as far. Try to mix up the routine by going different directions, different duration, getting on and off while away and at home. Like anything, horses learn by repetition and in this case to be away from other horses and not panic, but be careful to not establish the exact routine that will allow him to become disrespectful. You can use this principle while riding with other horses - move away from the group then come back even if the distance at first is 5-10 ft. Do as above. If you can sense the attitude of the horse and it has not developed bad habits, you will eventually be able to solo comfortably.