I deal with a lot of clients who telll me there horse is fine when out riding with a bunch of horses but get them out on their own and they come unglued. This is referred to as being herd bound. Horses become herd bound because they lack the confidence and/or the desire to get out and accept you as his leader.
First thing you need to do is establish yourself as a leader on the ground, read any natural horsemanship book and it will give you all sorts of ways to do this. When that trust and leadership has been established on the ground establish it in the saddle, again taking control of the horses feet is the way I like to do it. Once the horse has gained your trust and respect they will respect and trust that you will not lead them to harm and follow you. This all sounds easier said then done and it is. Alot of work goes into achieving this. I will give you and exercise to try when you are out with a friend on your herd bound horse. Ride together for a while then ask your friend to ride ahead 25 yards, your horse will start to act up but don't let them go with the horse until you say so. Once they have settled meet up again. Now you ride ahead 25 yards. Continue to do this increasing the distance and the speed and direction. Soon both will be able to canter off in opposite directions without a fuss.
Remember the key to a good horse is wet saddle blankets.
Have fun and be safe.
Steve Wawryk