Oh God this is all so incredibly familiar! And depressing!
My mother literally does not know how to have a conversation - to her it means waiting for a gap while the other person pauses for a breath and launching into a parallel, non-related story of her own. No matter what you might be talking about.
I told her about my very traumatic miscarriage and she immediately launched into a story about someone else's that she'd heard about. Yes I can see how she was trying to find common ground but in all honesty in that moment when I wanted her attention and yes, her sympathy, I quite simply didn't give a rats ass about anyone else's situation from several years ago.
She will always turn the conversation back to her. Every. Single. Time. and the conversations are largely medical updates now which is so very depressing - nothing life threatening just routine but must be repeated at least 10 times.
I really agree with the intimacy deflection theory - though interestingly she will go on and on about how she feels about various situations but zero awareness or appreciation that this same situation may also affect the person she is speaking at.
And I 100% agree with the drain V radiator analogy - this is my mother to a tee - she is an complete and utter drain, and has been her entire life. The glass is ALWAYS half empty. I think it's the negativity that is the hardest to deal with.
As for the shrinking world - don't know if that holds in my mothers case. She has lived in the same place her entire life, never, ever had any interest in other places. She has always been obsessed with discussing people of her small town, even as a child I remember the highlight of her week, every week, was to spend the afternoon with her mother and they would spend HOURS gossiping about their neighbours and townspeople.
Perhaps that was the pattern, she grew up with that. But so did I and I have NO INTEREST in gossip and actively hate it. I moved away at 18 and have never returned to live there. I now live in a city 3 hours away from my home town.
One of my biggest fears is ending up like her.