Everybody who is urging me to do something misses the point that what my life lacks, critically, are other people. And you can't make people love you, like you, spend time with you or care about you
You can't make people do those things, but what you can do is make it so that they want to do them.
Looks and appearance, sense of humour etc. are all subjective. Not everyone who's coupled up is a raving beauty you know. Ugly people, disfigured people, unusual, unpleasant or difficult people find friends and partners who love them.
The one thing that puts people off, be it friends or potential partners, is unremitting negativity.
That said, living with such a negative attitude would bring anyone down.
I really do think you should pursue obtaining some kind of professional help OP.
Talk to your doctor in the first instance and see what they think they could do to help you.
For what it's worth, I had an unpleasant childhood and didn't go on to do any of the things that you think are the rough guide to how life should roll out.
Most people would think my life now is exceptionally boring and in truth, it is, but I choose not to perceive it that way and to find some satisfaction and some fun in something, however small, each day.
Unfortunately at the moment you're stuck in a self fulfilling prophecy and that's what needs to be sorted out.
Are you employed or self employed? do you have work colleagues? is your work satisfying or does it just pay the bills? What kind of home do you have, do you like it? You say you're busy as an explanation for not doing things that people have suggested, so what are you busy doing? or are you hiding behind a needless use of your free time?
People here can start talking over these kinds of things with you to see if there are any changes you could make to start leading a happier life, even really small changes, but in truth, you have to want to change things and be receptive to that kind of discussion.
One last thing. I say all that with the kindest of intentions and not least because from your early posts I assumed you were late fifties to sixties and not in your mid thirties. I'd just like to see you starting to enjoy something out of life rather than throwing in the towel.