I don't want to go to my girlfriend's christmas party
Why don't you want to attend her Christmas party?
She always takes to long to get ready.
It's understandable to be upset when you feel like you're not treated with proper respect, but its bad to not spend the holidays with your partner. Partner's not spending holidays together may become distant.
I want to hurt her and make her feel the same pain that she is causing me.
People are expected to be supportive of their partners when they host events. If you have problems with her you should tell her how you feel about it.