I remember that feeling very well, Hey, and I really feel for you.
After years of travel and adventures, I ran into health problems in my early 40s and life seemed to dwindle away into something small and boring.
Like you, I had no partner or kids. I really started to notice how hard it is to make new friends when most people your age are settled into family life or at least with a long-term circle of friends.
Also I was navigating big unwanted life changes including bereavement, end of a long relationship and moving to a different area. I had to reassess my life, see where the choices I’d made (often without fully realising at the time) had led me and how to make the best of where I’d ended up.
I’m now over 60 and, to my surprise, have never been happier.
It’s partly luck and partly having learned to value things I took lightly when I was younger, eg relationships —with friends and others, not only family or lovers, though I am now happily married, to someone I met through friends. Also, I have lost my urge to travel to remote places, excel in my career, do amazing things. Life is far more enjoyable now I’m no longer constantly pushing myself!
I hope everyone here makes it through the midlife blaaah and finds lasting happiness.
Meanwhile, I recommend new interests, hobby groups, good causes you can help without draining yourself. All the ways you can put fun and friendship back into your life.