Fascinating topic, and lots of interesting replies. I travel solo almost everywhere, save for a bit of time with fellow enthusiasts in the past, so I've plenty of experience to draw on here.
One particular trip came to mind in 2018. In the early portion of that year, I was on the build-up to my first solo trip across the Atlantic, to Toronto. A very different mission objective back then, and all my previous trips of such a distance had been with company. Short-haul around Europe, not an issue doing that solo. Anyway, for a while I had been considering cancelling the whole trip, as I didn't much like the idea of the long nights on my own.
Thankfully I eventually sorted that out, and I had a good time. I would cut out the beers if I did the same trip again, but it's part of what I did back then. Overall the trip was a success, and less than a year later I did my first ultra-long-haul trip to Sydney. I remember that trip had the concern of being a very long way away from home, and the long flights I was concerned about being without company for. Fortunately, again I got past that and the trip was a success, although the long flights were still incredibly boring after a while.
As for hostels, I'm done with those. My trip to Sydney was in hostels, and I honestly couldn't do that again. I like my comforts like a bed in my own room, a lockable door, my own space and all that. 15 years ago, yes I enjoyed hostels but that scene is long behind me. Apart from a select group here, I don't tend to socialise. I have very little in common with most people, and I've grown to love my own company.
Indeed, most of my trips are more active than anything else these days. Sitting around in one place for all that long, it takes a level of patience I don't often have now.
Someone else mentioned Santorini in this thread, and I too have desires to go there. Admittedly, this was first sparked by a good friend who intends to go there with her family when the world returns to a more normal state, and I eventually looked at the island more seriously a couple of months ago. It looks just like the sort of place I could enjoy, luscious scenery and not too big a place to do walks and cycle rides to explore the island. There's even direct flights with Jet2 from Birmingham, which is quite handy for me, although to be honest I'd probably go from Heathrow given my love of that place.
Which actually leads me onto an interesting point. I love exploring an airport and noting down plane registrations while I'm there, particularly if I've got a good coffee to enjoy leisurely at the same time. The regular moving around the terminal is good for keeping occupied, essential to keep boredom at bay, and for exercise too. I do also love the people watching bit, where I've set up camp on a seat with my Favourite Flying Tunes playlist on or similar playing through the earphones. It can be quite interesting just to sit and wonder where someone's flying to, why they're running through the airport, that kind of thing.
I've been determined, since last year, that my first destination when I can safely resume my World Frenzy series is New Zealand. There has been a bit of a fascination for a long time on that part of the world, and whenever I read about the activities available on the South Island, or whenever I look at photos of the scenery, I get the urge to go exploring. I can't do that trip with someone else, no thank you. As far as I'm concerned, this will be an adventure holiday, and I have no desire to have someone tag along.
Granted, it will involve some sightseeing too, I plan to spend a couple of days in the Auckland area when I first arrive after all, but by the time I get to the outdoor activity bits I won't want to wait for my companion to get up and ready. Or for them to dawdle over breakfast or whatever. Generally speaking I'm an early riser, so by the time the sun is up I'm wanting to make the most of daylight hours!
This time I'm not even concerned about company on the long flights, and one routing I got recently was Heathrow-Doha-Auckland on Qatar. Well over 18 hours on that flight to Auckland, which would be a challenge! I'm not sure how I'm going to handle such a flight, I will find a way by 2022 I am certain of that. Already chosen to fly with BA or Qatar, the former tend to use 777s or 787s to Sydney which suits me incredibly well
Indeed, BA have flights with a stop in Singapore, and given I'm led to believe that is for refuelling it would also mean disembarking the plane for a short time, that option does sound ideal for a proper stretch of leg muscles.
What I'm getting at is that you 100% should NOT be afraid of solo travel. It is one of the most freeing experiences you can enjoy. I often get people telling me I'm brave to travel on my own or whatever, and perhaps it is the first time. I honestly can't imagine doing things any other way now, and until you've woke up on a trip with someone else, and you've got to be quiet while they sleep, while you're keen to dive into the shower and get a coffee made...Only once you've endured such an agonising wait for your companion to get ready will you understand the joy of solo travel!