A piece of advice I was given was "look at where they are now" around 3/4yo there is a lot going on with mental capacity & it's a Western society thing to have children sleep in their own rooms, it's not an evolutionary thing. Her fears of being alone are evolutionary rather than her being difficult or even that you have her sleep hours wrong. At her age she will have developed a feeling that she is independent of those around her, but also a realisation of how much she can be afraid of because the world is so big. Little things like a parent being in another part of the house & them calling out for the parent & the parent not hearing for 20 seconds can magnify fear, not being able to see a parent for 5 seconds in a public place can add to it. The fears of being alone can be very real & very scary.
For me I found that time was hard & ended up with one of my boys having a "camp out" spot on our bedroom floor. It was a quilt & pillow on the floor (not too comfortable) & another quilt for warmth & if they woke in the night fearful they'd come in, sleep there, it lasted less than 3months , but it was a lot better than having to get up to them. We noticed much the same happening with our DS2 around 3.5yo, but were able to solve it with the boys sharing a bedroom.
Often with the issues young children have a solution is better when we look at why it is happening, rather than why we don't like what is happening & how we can stop it. Have you asked here what would make her less scared, as if you've been telling her "it's ok" - it may be making it worse, because clearly in her head it isn't ok otherwise she wouldn't be feeling the way she is, the trick is more to listen to what they say & show empathy & help them find solutions. As an adult there is nothing more frustrating than someone telling you "don't worry" when you are worrying, but when they say "I hear you" you feel understood & it does help, same goes for children.
