One is taught (as kids) it is dangerous alone, just as mountaineering and hill walking are. It is in case something happens to you. But as an adult, if you hack out alone it simply means having a charged up phone, someone to ring and letting someone know your route or expected time of return.
I dont totally agree with the nerves side. I was scared to canter in the school - till I was taught how to do it in tiny incremental stages. Both horses and people need lessons in how to hack and it is best for the rider, I think, if you learn on horses that are used to hacking. And if you learn to hack riding first, i.e. being lead horse, in company. That feels quite like riding alone. Especially if you are trussed into a bp like I am and find it hard to turn and look back.
It is true the horse is a herd animal and needs teaching to go out alone - but there are benefits of riding first or riding solo - Your horse is picking up the cues from you the rider and not getting excited by following or challenging the horse ahead. The other benefit for me as an adult, is that I like to control my own vehicle - eml has written about her qualms when being lunged by someone. I dont like handing over control, so I always preferred leading the ride. I used to lead for a purpose, to introduce new horses the routes, or since I was on a safeish horse, to lead if we rode past spooky objects and the other horse hesitated.
Anyone scared of going alone, I really recommend setting out with another rider, but asking your horse to lead the ride. The worst that can happen if you are leading, is the horse can spook and you can pretty much watch for warning signs of that - head position, and half halt and in canter using more leg.
And if you are leading or alone, you set the safety margins. I never trot or canter when loose dogs are around. Nor if a stag is close to the track - Allow horses to take time to observe and accept half hidden animals or unfamiliar objects.
I find it much easier to use riding to reassure myself than to mess with psychology. I fall off a lot - and want to minimise that. I expect any horse I hack to transition in and out of canter at 20 strides, exactly. Plus never to transition up or down (down is important) unless I ask. Rashid has this great thing about knowing where your destination is. When you cue the trot or canter your mind should be clear how far you are going in that gait. That's why I (now) like dressage tests so much - there is no open ended charging about.
If you calm your nerves, the danger is that you wont be riding with full care and attention. Driving a car does get easier with practice, but you still need to give it your full attention and concentrate. My last fall from Maisie was because we were idling along in walk, chatting and the horses took exception to a yellow dredger. She had passed similar before, but I would have been riding her more positively. My recent falls have come because I wasnt paying 100% attention. Relax too much and it is easy for hacking to seem simple. You dont ride round a course of jumps or a dressage test chatting to someone and abdicating control. Same goes for hacking.
And then there is the other thing. If you dont like jumping or hacking or whatever, simply dont do it. There is no moral obligation to risk your neck doing something you dont like. OH doesnt much care for hacking. The reason I tackled my fear of canter in the school was I need to canter horses that I am looking to buy or share.
If like many of us and Owned by Chanter, you think hacking is good for horses that's fine. But you could always find a sharer to do the hacking for you or pay someone. If your horse hacks regularly on its own ridden by someone else, then it will be easier for you to take over and even enjoy it yourself.