The only reason I brought up children was that I haven't had anybody tell me that if I'm not married or not Christian I'd feel empty and lonely when I'm old, but I HAVE had someone tell me this about not having children. I think it's all BS that you will feel empty and lonely when you are old if you don't follow the typical path in life. Everyone must follow their own path and for a few of us, marriage and family and God aren't the path. It hasn't been my path.
I go through periods now and then when life seems flat and empty. At this point in my life I now see that this is always what happens just before everything changes again. It's a necessary step. It sucks and when you are in it, it seems like it'll never end. But it does end. So I now just embrace and accept it and know that I'll soon be off on another adventure.
I took a really good class once where we did lots of exercises to try to unlock our deepest longings and callings. We bought one book for the class called "Callings: Finding and Following and Authentic Life" by Gregg Levoy. I highly recommend this book. It was not just a career-oriented book.
While reading the book and after doing the class it suddenly hit me that what was missing in my life was to go hike the Pacific Crest Trail, something I had always wanted to do since I was 10 years old. It turned out to be the most authentic experience I ever had, one of the most important things I ever did that actually "made the world a better place". Without trying, I touched so many people in so many ways out there I began to see that my hike wasn't even about me. It was something I was supposed to do to be a catalyst for a whole bunch of total strangers that I met along the way, a catalyst for change in their own life. It was an extraordinary thing. And I never would have gotten there had I not been really super miserable and empty and lost for about a year.