from recent experience i can tell you that my first fast of seven days (about a month ago) was a nightmare of anxiety... all kinds of anxieties, you name it... some related to fasting, some not at all.
but my second fast (of which i am currently on day 12) has been fairly free of anxiety. i really do think it gets easier the more you do it. if i were you, i'd follow Fonty's advice and just do short ones and build up until you realize that your body is your friend -- it will take care of you and do miracles for you if you just trust it enough!