the added self-imposed guilt that Mums should be the Most Important Thing on my list of worries

Isn't family weird.

My mother can't cope with the notion that she isn't the most important thing in my life at all. I think it's a side-effect of having given up her own independence and identity; she let me take over her own life, so in her world I "owe" her my everlasting devotion.

To me it looks more like she made a duff decision about how to look after herself, really.