Every time my fiance has to leave town this feeling of dread comes over me.
I'm sorry to hear that. Why do you think you feel so much dread about your fiance leaving?
I don't like to be alone.
It's pretty normal for people to dislike being alone. We're social animals. It's particularly understandable to dislike separation from a partner.
But it's making it difficult for me to function when he leaves.
If it's impacting your life that much then it might be a good idea to talk to a therapist about how you're feeling. While it's okay to be sad when a partner is out of town, it shouldn't be so overwhelming that it makes it difficult for you to carry on your life.