Going solo is great for some, and quite daunting for others. But if you don't try it at least once, you'll never know! For me (late 60s) it was a huge learning curve and like OP I was scared witless of doing it. And it does get easier as your confidence grows. It's never QUITE the same as being with others, but if there's somewhere you want to go and no one else does, you have to do it! Life is short.
Again, like OP I wasn't comfortable around couples and families so I made sure I went to places that were not "fly and flop" type places, although I love a few weeks R+R with DP just the same.
So I did things that interested me where it (I hoped) didn't really matter that I was on my own. The most recent was a trip to the WW1 battlefields in Belgium. A distant relative was killed there and it had resonance. I travelled to Brussels airport and immediately took the direct train to Ghent, which was a fabulous place in its own right. Arranged a battlefield tour from there with Flanders Tourism or something like that via the tourist office in Ghent. Then went to Bruges and back to Brussels to get home. Train travel was really easy and I did it and thoroughly enjoyed it. I used the Trainline app as they compare all services for times and prices and you can book where needed. Others may have different suggestions. Met many like minded people in Flanders Fields too.
If there is one piece of advice I can give to solo travellers it is to travel light! I only use an underseat size backpack for a few days away. It is so much easier getting on and off aircraft, trains, buses etc. with just a small bag on your back, and it's easier to keep it safe too. If only going to one place then it doesn't matter so much, but if travelling around, that's what I do.