Please Help! 7 year old that has nightmares.

Today makes 3 weeks since my daughter has started to get nightmares. She doesn't wanna sleep in her room and when she does all she does is cry. She doesn't want to sleep unless someone is with her. However she recently switched rooms which she choose too because she wanted the big girl bed before her little brother gets here( currently 4 months pregnant) . Please give me some tips on how I can get her to sleep by herself again. Because this momma is wore out and my bed is already feeling cramped.