Dear Heidi,
Looking back, it would have been better to have allowed your daughter to remain in her pre-school. However, that is now water over the dam, so to speak. In light of the family's history, it's not surprising that your daughter is worried about absence from you. She will learn, from experience, that separations such as those prompted by school, will be tolerable. But, she'll need to learn by the experience itself - it's a matter of over-and-over-again type of learning.
Try now to locate a daycare or pre-school experience, prepare the provider or staff by noting past experience, and be resolute about having your daughter attend. There's no casual or easy way to solve such dilemmas. You have to be ready to bite the bullet. If you can, your daughter will adjust.