Planning a wedding is probably one of the most exciting and (perhaps) stressful things you will do in your life.
My advice...kindly tell your friend that you are disappointed that she is no longer willing to share your special day with you, that you are sorry that she is afraid to fly, and that you wished she would change her mind. Then move on and continue planning the most beautiful day of your life.
Don't talk to her anymore about your wedding details, as this will only make it harder for you to accept that she is not coming if she shows no interest in what you are telling her. Ultimately your wedding day should be all about you and your fiance and no one else.