I want to revoke my attendance at this bachelorette party completely
Why do you not want to attend at the party?
I do not wish to go.
It is understandable not wanting to go to a bachelorette party but it is wrong not to support the person whom the party is for. You should attend the event that you were invited to, just for their sake.
I can't.
I still think that it is best for you to attend, because this event is so special for the bride to be, and it will hurt them to miss out on your company.  However, if, in addition to not wanting to go, you are also incapable, then you should let the bride know that.  Let her know that you can't attend, don't leave her thinking that you don't want to be there.