Horses are herd creatures and feel safer in company than alone. However, it is not always practical to ride out in company and you need to be able to rely on your horse when alone.
Some horses take their riders to a certain point and insist on turning for home and some become scared of traffic - especially heavy vehicles. These are dangerous situations for both horse and rider and it is important that you overcome your horse's aversion to hacking alone.
To begin with, try a short journey. A ten minute walk with a return point chosen by you would be a good start. Don't expect your horse to be happy to spend hours alone if she craves company. The time out alone needs to be built up gradually.
Make sure you take a stick with you and be firm with your commands. Use voice, leg and hand commands simultaneously and back-up with a tickle if your horse doesn't respond.
Never let your horse decide where and when to return to base. This is a serious breach of behaviour and will result in further misbehaviour. If your horse stops and refuses reasonable commands to go forward, stay put. Hopefully, she will eventually get bored and move on. Be prepared to wait for as long as it takes and when she moves on, even for a few steps, praise her generously. Don't push it - turn her for home after she has obeyed you and moved forward.
If you feel she will spin around and use her superior strength to overpower you, arrange for a friend to accompany you on foot and get her to lead your horse when she naps.
Be warned, it takes time and patience to deal with problems and if you try to rush it you will generally find it takes longer. Your horse may never be entirely happy to hack out alone and you will probably find she will be a little more skittish when alone and you will need to be aware of potential "monsters". However, it is possible to take her out alone as long as you take it easy, remain in charge and praise progress, however slow it may seem.