OK, firstly I totally understand that this is hard , really really hard. Cut yourself some slack. But you can’t walk away.
If she is ND, and it sounds as tho she is, then standard parenting advice re rewards and sanctions will not work, because to an extent the behaviour is out of her control at this stage. (Control will come as she matures).
My daughter was the same and now at 11 is diagnosed with ADHD, autism, sensory processing disorder and PDA. I won’t lie, it has been extremely tough. But the moment we read extensively about parenting an ND child, things changed. So no sanctions or rewards. Connected consequences - so leaving from day out but calmly - come on we are going home, this isn’t working for you atm. Removing thrown food but not refusing to provide (simple) food half an hour later. Many ND children cannot recognise hunger or thirst. Keep a diary to look for triggers - noises, movements, smells, touch, fabrics etc. Our DD could not articulate the impact these were having and it would manifest in her behaviour. Now at 11, she can, and we work with this - so no themes parks or fun fairs, but quiet beaches and countryside. No cinema or theatre, (too noisy) she still doesn’t really watch tv. Books are good but we would accept that she won’t cuddle and look at pictures, she will hang upside down, do roly polys etc.
She can’t manage places with strong smells and any sort of eating out is challenging. Picnics are good. She wears very specific clothes and generally has the several of the same things.
Meltdowns - acknowledge she is struggling and then ignore if low level. If really kicking off, take to room and stay inside with her so she can’t trash the house. She can trash her room. If violent - time in or not violent restraint. Not cross - ‘it’s ok, I’ve got you, I’m here, we’ll get through this.’ Etc.
lots of ways to burn energy in her room - yoga swing chair, ours has a small inside trampoline and a doorway swing/pull up bar, and a birthing ball! she can’t manage clubs or team sports but she does boulder at a very calm, mainly adult attendees, bouldering club.
My DD masked in school and was v quiet and wouldn’t talk about it, meltdown as soon as she left. We did home ed her for 18 months from Y4-6 because it was just too traumatic for her, but she is back now and starting comp in September with SEN plan and reasonable adjustments. But it has been a huge fight.
Oh And she is now prescribed melatonin for sleep which has really helped, she is asleep but 11 now, not 2. And calmer in the evenings.
it is really hard and it still is, our lives have changed a lot. On the upside, we could stop soft play and trampoline parks (definite no go). We do a lot of tag team parenting with our other child so he doesn’t miss out. Different weekends away with one or other parent so they both get 1-1 parent time and DS gets a break!
She is amazing now, and is largely able to articulate her feelings. She is sometimes frighteningly mature, and at other times she is 3 again, but reflection is beginning to happen. She has one friend. She is a joy most of the time, and meltdowns are generally brief because we all understand how to manage.
i hope that some of this ramble helps. Take care OP.