Oh gosh, I know how you feel! I recently went to a big birthday party and didn't eat one thing there. I ate before we went and I carried around a glass of Crystal Light and an empty plate (I know -- sad) but I felt a lot more comfortable and survived the party!!
When it comes to birthdays for my husband, son and daughter, I allow myself some cake. They are 5 and 2.5 and having cake is a big deal so I give myself the ok and then get right back on program.
Sorry if none of this helps. The fact that you are planning on how to deal with the situation ahead of time says a lot about your commitment. You'll do just fine. JenB72 said it just right: "If you decide to indulge in the goodies, you need to accept responsibility for it and the results and feelings that come from it. Whatever you decide to do, just remember it's your choice."
Have fun at the party! It will all be ok