Ask her why she's scared, if she can't tell you verbally use other methods like drawing, to understand why she's so scared. When I was kid I used to fear falling asleep alone, because I hated to sleep when I was cold, it hurt my bones and made me feel like I was going to die. I feared that if I slept without heat next to me, I'd lose heat and die in my sleep. Granted as an adult now telling my mom way I forced her to sleep next to me was... I understand why that was unreasonable, however, as a child it felt real to me. Then work on ways to make her as comfortable as she can be so she can sleep. Maybe try out different things, like for me, I have to be surrounded by stuffed animals. Feeling something against my back helps me sleep. I also need a thick blanket, so figure that out for her. Try different things, like using stuffed animals, or blankets, or one of those sleep sound things that makes static noises to relax, or even smells. Use her sense to comfort her, by understanding what those comforts are. Instead of making it the enemy, make it a tool to help her. When she gets older things might change, but for now do what you can to comfort her at her age.