A lot of my isolation is situational -- small town, single mom, family AWOL -- but I've always been on the edge of any social group I've been part of, find it bewildering and exhausting to be in the middle (which hasn't happened often). My interests are very different from those of other single moms I meet; the only moms I meet who share my interests are far-better-supported than I am and don't really have time for me anyway, I'm not in their crowd. Old non-parent friends didn't really want to deal with the social constraints of my having a child. In other words, people go on with their own lives and forget me, mostly. Usually I don't have time for socializing anyway. I saw more of the moms when the kids were little and we all brought them to local activities, but the older-kid activities seem to involve crazy commitments of time and money, a lot of driving all over the state in minivans. I don't have time or money for that sort of thing.
I tried LD online dating for a while, but one after the next attracted brilliant, handsome, and deeply troubled men, well-acquainted with getting thrown out of places. One felt like a long-lost twin; came and stayed with me, then went home and killed himself. Another has enough trouble with AS that finding a steady home and job seem insurmountable troubles.
I've no idea what they think of all this at work (the isolation -- I haven't told those stories). My default is to assume that nobody's interested enough in me to gossip, but that usually turns out to be wrong.
How have I adapted to it? By being sad, I suppose. I'll have more freedom to socialize in a few years, and more freedom to move to an amenable place, but I get the impression that late middle age is like 10th grade -- by the time you get there, everyone's all cliqued up, bound into their families and longstanding social circles. I guess it'll be more drifters and other troubled people who never found their way in, and a lot of solitude.