This is clearly distressing and distracting you.
If this were a boyfriend we were talking about, you realise he'd be described as clingy and needy at best, but more like a demanding, controlling, coercive, emotional manipulator.
Trying to take every second of your attention and time, using your energy as his only source. Just waiting for his next fix.
And that's a good way to describe a parasite.
Are you seeing it yet?
Right. Get off Mumsnet. Text her and say something's come up and you won't be available after all.
When she asks what, do not answer the question.
Wish her a nice day shopping, whatever. When she says she's not going if you aren't, don't answer.
Your energy is needed for yourself, your family & home, your job. Stop feeding the parasite.