I'm having a middle of the night panic about our impending move abroad!
The guilt/worry about our parents age and health concerns me. They are all ok now (early-late 60s) but I'll feel so dreadfully sad if their health declines. And they are going to miss our DC so much 
The coming back bit worries me. My DC are 5 and 2 so if we live away for 2/3/4+ years will it be disastrous for them when we come back in terms of social stuff and friends or academically (they will go to French speaking school in Switzerland)?!
My irrational panic is telling me they will feel 'different' to the other kids and not be up to speed with all the games, fads, TV stuff, learning, rules, ways of doing stuff and will end up being bullied or rebelling and never feeling like they fitted in and it will sod up their whole potential will be damaged by the whole experience.
The idea of it being a great adventure and fab to learn another language and all that is all very well in theory but now it's coming to the crunch, it all feels a bit whimsical and scary.