Blue - I get like this before holidays too, I'm fine once I get there but the packing and travelling I find really hard. I would suggest breaking it down into baby steps, go and put some washing on, then maybe get your bags out, have a little break, sort out something else. Leaving it all till last minute is worse and you always end up forgetting something IME.
Undercoverbanana - yes, it sort of makes sense, but really you are more important than that. I used to feel a bit that way before I had the children, what was I doing it all for, I also feel down when we reach a change of stage, such as my youngest going to secondary school recently. Obviously I don't know anything about your children, but I would imagine you are still a very important part of their lives, even if they are away from home and independent now, I grew much closer to mine again from my 30s onwards.
Reike - I hope you're ok today, it sounds as though your boss is reasonably sympathetic and will be OK even if she does figure out the hours thing. I guess these things can never be planned completely anyway because anyone could go off ill at any time, especially at this time of year with so much cold, flu type stuff going on.