I don't want to go to my birthday party
Why don't you want to go?
I don't want to see people from high school again. I just don't feel like it.
It's okay to be uncomfortable with parties. But you should go if it's a party being held for you by friends. They care about you.
I would just ruin the party with my sour mood.
Even so, not going to a party thrown for you is wrong. You need to at least show your face.