I think, you should really cop it sweet... he's cheated on your cousin with you, he's cheating on you with your cousin and god knows who else.. and once a cheater always a cheater, so your both better off with out him.
What you did was completely wrong... your 18, you know right from wrong, as does he.
How would you feel if she had done it to you.???? How would you feel????
I wouldn't like to carry around that secret, and feel sick every time I see her....
I'm not coming down on you personally, its just the "Act" that is up setting.
What ever you decide, I hope it works out for you... but you do need to get away from him...