I know exactly how you feel! I used to start dwelling on the pain of leaving, even before the fun had started! It was awful! I finally realized that's just not productive. Now what I do is consciously try to savor every moment and store it all away in my memory and in my family's memory as much as I possibly can. That doesn't mean I try to pack as much activity into every instant as possible, or that I never sleep! I just take way more pictures than I used to (not just of big things like "us standing in front of the castle" but of the little things like the ice cream cone we accidentally dropped on the ground and all laughed about, and so on), I journal as much as I can, I get the kids to tell me little details they want to remember, I make a nice memory box afterward for all the little mementos. I try to focus on capturing the happy feelings and storing those forever.
We've started taking pictures with our minds, too. We will sit in one spot, say on a bench at the beginning of Main Street, and just take in everything around us--sights, sounds, smells, feelings...if you really really observe with all your senses, you can capture that and it will stay with you for a very long time. That helps things last too.
This all sounds silly when I write it out but it has helped me defeat the pre-trip blahs you described. Well, at least most of the time (I still can't help myself from thinking, "oh no, that'll be us in a few days!" when I see people checking out of the hotel when we are checking in).