I often get a growing feeling of anguish and discomfort and why-did-I-put-myself-into-this-perhaps-tomorrow-morning-will-be-a-day-like-all-other-days-ever on the day before leaving home for someplace else. It's like preemptive home-sickness. This occasionally spreads to my familiars. This typically stops once I do leave, however, so it appears to not really conform to the definition of homesickness I found on Wikipedia:
Homesickness is the distress or impairment caused by an actual or anticipated separation from the specific home environment or attachment objects. [...] Homesickness is especially common in youth. Young people may experience a sense of dread, helplessness, or separation anxiety on their first day of school, summer camp, or on a protracted summer vacation away from the family.
(Emphasis is mine.)
Does this feeling have a name?