I think there was some pretty good advice in that thread and I don't have a whole lot to add. I'm around the same age and a lot of what you wrote feels very familiar to the me of my 20's; the last five years I ended up doing the full blown relationship thing which tends to remove a lot of these kind of issues and replace them with a others.
One thing you do appear to have is a lot of freedom - freedom to spend your time how you want and go where you want, without being answerable to anyone. That can be a mixed blessing but it's something you can use and something you might well miss when you no longer have it. Part of me would love to be living on my own in America!
This is basically the 'quest' thing again, but one thing that often helps when I'm in a rut is to find something with a tangible objective at the end and succeed at it. This is especially good if it's something you're naturally quite good at in the first place, because you'll most likely find it easy, and it feels good to know you have skills and are making use of them.
Like for me, I spent a couple of months recently doing a video editing course at the BFI, cutting together shorts and doing a couple of filming projects. It's the kind of stuff I did at university and still do at work to some degree, but I got a massive buzz from being able to use the skills creatively again. And doing things like that makes you want to do more things. This kind of thing can help with your career as well, if only because it gives you stuff to talk about in interviews.
In terms of socialising, I don't know what Meetup.com is like these days but in my single days I used to use that to keep myself busy. You've read a million books so maybe join a book group or something?
I think you will be okay. Also if you're ever in the UK you should totally come and hang out with me and A Smith!