Sex with my cousin

So two years ago i had sex with my cousin, i didnt want to but he preasured me into it and was like dont tell your mom or anyone.. Or he used to say dont ever let anyone cum in you i want to be your first or he would tell me he was my boyfriend and would beat the shit out of anyone i ever talked too. Whenever we would go to bonfires together he'd tell everyone im his gf when really i wanted to just come out and say im his cousin.. Any time he'd text and say "do you wanna hang out" id tell him im busy because i dont want to have sex with my cousin. Im 18 now and engaged to my best friend and want to tell him but i dont know if i should because its in the past and before we even got together. But i feel like if i dont tell him and he found out in the future he wouldnt trust me. I dont want him to think differently of me because its gross and incest is an abomination and a sin aswell. I need advice, please dont be mean to me ive never told anyone because its quite embarrassing..

He hasn't bothered me in a year he has a girlfriend now.