The stresses of parenting can be torturous sometimes. My heart goes out to you!
I imagine you've already addressed what you, your husband and home mean to your daughter and I imagine words such as 'security, reliability, peace of mind and reassurance' have come up in conversation. Of course, for any of us, we need proof that we are self-secure, self-reliant, personally at peace and self-assured. To prove this to our brain (which can be stubborn at times), we can try strategies that trick it into believing independence is possible, until it is able to form a new and positive outlook. Wondering if a kind of 'weaning' strategy has been talked about. With so much technology at our fingertips these days, have you discussed something like Skype being a possibility? Would she be able to stay at a trusted friend's house, with the knowledge that she can Skype with you whenever she wants? Might be a bit tiring for you to be doing this on and off continuously throughout the night but if it turns out to be something that sets your daughter on the right path, it will be worth the initial sleepless nights. She could try doing this one night each week, with her and her friend having an anxiety management plan in place. Gradually you could alter the routine in a variety of ways, in the process.
An outside the square type of strategy may have her seeing herself with a gradually changing identity. 'I am someone who takes responsibility for my personal security, whilst seeking a sense of security from Mum. I am someone who's self-reliant, whilst still being able to turn to Mum. I am someone who shares a sense of personal peace with Mum (aka Mum is not solely responsible for my sense of peace). I am someone who is assuring myself of my capabilities whilst receiving support from Mum.' The I am part is key. Our I AM is our identity, it's how we identify ourselves with life.
The Skype sleepover is just one idea. Discussing this idea with your daughter might help her to start thinking outside the square regarding possible pathways to greater independence. Of course, the Skype thing is simply a tool in a process, not intended to be something she becomes addicted to or fully reliant on in an unhealthy way.
When reflecting on when your daughter was a baby, you will recall that weaning can be a slow process. Whether it involves changing from milk to solids or changing from dependence to independence, strategy is what eases us into change.
Take care of yourself