One reason for being lonely at the end beside being autistic (in my case - but I think autism is much more outspread than we think, as being tone deaf, or left handed) is that with aging you build your defended burrows of experiences, interpretations of these experiences, skeletons in your closets, memories of ecstatic moments and agonies (more agonies than ecstatic moments), important for you, but incommunicable to others. The whole thing becomes an impenetrable fortress, where you cannot admit anybody if not at the risk of having everything crumble and be forced to live (or die) under the waste.
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Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
--Samuel Beckett