I'm going to leave my birthday dinner
Why do you want to leave your birthday dinner?
I can't stand the pricks.
You should stay for your dinner despite your reluctance to attend. In the future, communicate to the people that planned it for you and reach a compromise or even disallow it.
I can't handle it. I don't sleep well and I'm only going to drink. They are the worst.
I still think you should try and hang out with them since you've already planned to do so. It'd be pretty rude to leave early.