That sounds incredibly, incredibly hard – I'm not surprised your mental health is suffering.
Firstly, do you and DH both get a break, some proper downtime, even if it just an hour to go out and get a coffee and meet a friend/read a book at the weekend?
What does she like to do? Is there anything that makes her happy or calms her down? Bouncing on a trampoline, swinging, spinning, cuddling up with a blanket and teddies, music, fidget toys, even screen time – it's very individual. I'd try to build those things into her day at regular intervals, part of her routine to help try and keep her in a better place emotionally.
On the other side, are there things you know she's always struggled to cope with, certain noises, clothes, textures, specific places or activities? Try to find workarounds for as many as possible.
Have you and her teacher spoken to the Senco? Is she getting any support in school, as she's clearly struggling? Support is based on needs, not diagnosis, so a referral is good, but they should be putting things in place to help her now.
It might be worth reposting this in Special Needs Chat or Special Needs Children. The boards are a bit quiet, so it might take a little while to get responses, but posters are generally very helpful once they get time to reply.