"I have been getting it wrong for years!"
Oh, I doubt that's true. And that kind of thing can happen to anyone...honestly. And, to be honest, I think it was perfectly reasonable of you to expect this man to have remembered the daughter of his oldest friend! That he didn't says vastly more about him than it does about you.
The most difficult thing about loneliness is believing that everyone else is having a happier time than you - and that reinforces the idea that there's something intrinsically wrong with you, that it's all somehow your fault. "If only I were different - funnier/cleverer/wittier, whatever, I'd be out there having fun too".
And it's bollocks, of course. Loneliness is everywhere - in the most unlikely places, and afflicting the most unlikely people. I remember reading an article about the death of Marilyn Monroe, and someone commenting that she had no one to call on the night before she died - the most desirable woman on earth had no one to call (other than people she was paying).
It would be easy to say - right, Ice off you go and join a Salsa club or the local reading group, make yourself some new friends, but that's a near impossibility if you have social anxiety, so pretty useless advice, really.
Baby steps is what's needed. Start small and leave the big leaps until later.