Hi, I'd like to help you with your question.
It sounds like the story she read about slavery tapped into a fear of separation. Most children go through fears and it is completely normal. The fact that she is able to go to school without you is very positive. And her fears can be treated relatively easily. It is good you have an appointment for her already. Therapy will help resolve the problem. In the meanwhile, there are things you can do to help her.
One, make sure you are consistent with telling her to sleep in her own bed. I know it is hard when your child is afraid because all you want to do is comfort her. But the only way she is going to get over the fear is to consistently see that nothing happens when she sleeps on her own. Repeated affirmation that you will be there in the morning and that nothing happens at night to her will help her overcome her fear much quicker.
Two, be out of her sight for increasing periods of time (leave her with a sitter or other trusted adult). She needs to know that just because you are out of sight, that does not mean she will lose you. Start by leaving her for 1 minute, then 5 then increase it slowly until she can be alone for as long as she needs to. Each time, reassure her you will be back, but don't make a big deal out of it. Don't let her cling to you or carry on. Look her in the eyes and tell her you are coming back. Then leave. She will see that you do come back and her fear will lessen.
Three, praise her when she is able to deal with her fear without becoming too upset. Tell her you are proud of her. Take everyone out for ice cream (include everyone because you do not want to single her out for special attention and cause sibling rivalry). Tell her she is brave to face her fears. This will help reinforce her facing her fears and coping with them.
Here are some books that deal with childhood fears:
Freeing Your Child from Anxiety: Powerful, Practical Solutions to Overcome Your Child's Fears, Worries, and Phobias by Tamar Ellsas Chansky
Anxiety-Free Kids: An Interactive Guide for Parents and Children by Bonnie Zucker
The Anxiety Cure for Kids: A Guide for Parents by Elizabeth DuPont Spencer, Robert L. DuPont and Caroline M. DuPont
You can find these on Amazon.com or your local library may have them for you.
I hope this has helped you,