Our DD had sleep anxiety from the time she came to us at 19 months until just before her 5th birthday. We were ALWAYS miserable. Never got a decent night's sleep. We finally were referred to a sleep specialist because we were at our wits end!!
The program she had us follow was strict and had to be done a certain way. It was a long process and it was not fun. But at that point we were willing to try anything.
I don't remember the exact regimen, but it was something like this:
She always slept in her bed. No exceptions. The other rule is that whichever parent starts the process has to be the one to follow through to the end. No taking turns.
For the first week, DD's mattress was taken off the bed and placed on the floor and the parent would sleep on an air mattress (or a seperate mattress) pushed right up next to her mattress.
The next week, her mattress was placed back on her bed, and the air mattress stayed on the floor nearby. Parent still sleeps in the room, but now a little further away.
The third week, the air mattress is removed and replaced with a comfortable chair. You sit in the chair in silence while the child falls asleep. Then you leave and sleep in your own bed (finally! LOL).
The next week, the chair is moved to the hall outside of the room (or you can sit on the stairs, or anyplace near the room where she knows you're there but cannot see you. You wait for her to fall asleep, and go downstairs.
Then at last, you get to the point where you take her to bed and tell her it's time to sleep. You tuck her in and go downstairs.
I NEVER in a million years thought it was going to work, and as I said, it was NOT fun. But it worked!
It's very ritualistic, and there are other "rules". Like you say the same thing every night at bedtime. I think ours was "We are here, and we aren't going anywhere. We will keep you safe." Then it's a hug, tuck in, and no more talking or distractions. Any bedtime books or stories had to be read downstairs before going up. At bedtime, the bedroom is only for sleeping. Not talking or playing games or watching TV.
Of course we had struggles whenever we transitioned into a new "phase", but we were always calm and explained to her that this was all to help her get better sleep at night. And of course before any new transition along the way, we would sit down and explain to her what the next "step" was. For example: "Tonight, I will sleep in my own room, but I'll sit in this chair until you fall asleep." Or, "I will still be here while you fall asleep. I will just be right outside your room."
It felt like the process would NEVER end, and as I said, I questioned whether it would work. But in the end, it reassured her that we are there to keep her safe. It's been over a year and no problems since.
She was quite a bit younger than your DD, but I would think that it's the same concept no matter how old or young the child is. She just needs that comforting reassurance that you're going to be there if she needs you.
Sorry so long, but good luck OP! I hope you can get her sleeping again! I know how desparate a parent can be when it comes to sleep issues!
ETA: Just to provide a little extra background, our DD does have genuine anxiety issues. She has PTSD and abandonment issues. So you can understand why I doubted that ANYTHING would help with her sleep anxiety! But this method really did work.