This was me.
My story is, I'd suffered a bereavement in the summer, a grandparent who lived with us. My Mum wasn't great generally about making us feel secure - often threatening to leave, or scream until 'the police come and take mummy away', or kill herself. I also had an operation go wrong at the start of y5 so was off for a month. And I'm on the autism spectrum...so, a perfect storm.
I was convinced that mum was going to die, or leave me while I was at school, or that my heart was going to stop (which is how my gran died). I was a bit of an awkward child. Loads of acquaintances, but no close friends, so never quite fitted in.
I cried, genuine distraught tears before school each morning. Strangely, once I was in school, though, I was ok. I remember being quite happy by break time.
With benefit of hindsight, I probably had a spot of reactive depression resulting from my gran's death, and how my mum wouldn't let us grieve or talk about her. Then the attachment stuff.
My advice (eventually, sorry!) is CBT or counselling. See whether she can be helped to build up her own internal resilience. I was prescribed antidepressants (aged 9!). Really don't recommend that.
Be loving, keep a sense of humour if you can, and talk! Talk to her, and talk to others for your own sanity.