I'm right there with you!! When I made our reservations months ago I was determined to lose some weight. What happened? I ended up GAINING 15 pounds!!
My DH and I just went and joined a gym a few days ago. One of things you get when you join is a fitness assessment that includes a calculation of what your body age is based upon your weight, % body fat, and various fitness levels. Well I am 33 years old and my body age came back as 46!!! So the Disney trip didn't seem to motivate me the way I thought it would but finding out that I have the body of a 46 year old when I am only 33 definately got my behind in gear!!
Now my focus is on getting healthy, not just losing weight. For some reason, that has kept me highly motivated the last few days. Maybe that would work for you too. Instead of trying to lose weight before your trip, focus on ways you get get healthier - exercise a little every day, eat more fruits and vegatables, cut back on sugary snacks, etc. The outcome will be that you WILL lose weight, but you won't have to be so focused on the scale. Good luck!