Don't want to go to a party, can I say no?

Basically I have been invited to a black tie party for my cousin's 18th bday. My parents have also been invited. I have social anxiety problems and will not know any of my cousin's friends etc, and barely see my cousin anyway so I really do not want to go to this event. However my mum is insisting I do and saying it would be rude etc. However I'm 18, shouldn't I be able to make my own choices about stuff like this? The thought of going makes me feel like having a panic attack.