I'm really disappointed that my family is going on a trip over my birthday
Why are you so upset about that?
I wish they would make me the center of attention.
I get that you are upset, but sometimes plans just do not work out. Not everyone thinks that your birthday is as important as you do.
Truth is that they all just think they are too good to spend time with me and don't want me to ruin all their fun.
That sounds really ad you don't think that is true do you? I hope you still have time to plan something for a later day, I think you would be able to have more people show up if it is on a weekend.