Sounds like your loneliness comes partly from the state of the relationships you do have, Gordon. A person could have a million friends and still be unhappy if their romantic relationship wasn't quite right. Maybe you need to start by fixing that - or getting out of it.
It also sounds as is you're a person who enjoys have one or two close friends instead of dozens of pals. That's ok, imo. But it seems you and your 2 close friendships have been impacted by the fact that you are all at different stages of life. The married one, I guess, is busy with "married" things (activities with just their spouse or other married couples, etc.0 and so "not available." I'm not sure why you don't seem to be satisfied with your friendship with the single one? Is it because you'd feel better if they had a partner, too? Or could do more things together (double date)? Or is it that the single friend prefers to do things you don't do anymore (go to singles events, hang out late at night or whatever)? You need to see, imo, if you could bond more over a shared hobby, sport or other interest - something you could do together, just you two or maybe you two and the married friend. Make it about shared interests, not relationship status.
But if that won't work or it's not enough, then, yes, I agree with ninathenana, you'll have to find new friends in "like minded people." Perhaps some of those "acquaintances" would fill the bill? Perhaps you need to "look at" them again? Or take the "usual suggestions" that nina mentioned and go to a club or class or two. Your call...