It's generally impossible to talk people, whether they are big or little, out of their fears. It's far more helpful, in most cases, to deeply empathize so your sweetie has a chance to work her feelings through to where she can deal with them.
Some examples of things you might say, followed by empathetic "Hmm" and "Oh, I understand," and "I see/I feel that, too":
Oh, honey, you had a really sad and scary dream, didn't you? Can you tell me about it?
…Dreams feel really real, don't they? It's like watching a video, but you're in it, so it feels like it's really happening. Is it that way for you?
…What was the part that felt worst to you?
…Did it get better after that?
…You are really afraid of being lost, aren't you?
…Have I ever lost you? Besides in your nighttime dreams?
After she seems to have said everything she can about her fears, try looping her into more positive feelings. This will help bring her into the present, where she is safe:
…Is it better now? What makes it feel better for you?
…Are you feeling lost right now?
…How do you feel when you're NOT lost? Pretty happy, huh?
…Would you like to have a plan, just in case you ever get lost? What do you think you should do first? (Some gentle coaching here is helpful.)
I remember my mother trying to argue or scold me out of fears of the dark. It never helped. I use the above techniques with my grandson when he's struggling with any sort of negative feelings, and they seem pretty evenly effective.