How do I end my relationship?

My fiance has had a drug problem since I've known him. It's a hard drug not weed or anything like that, we have broken up a few times over it, hes gotten help and I told him if he does it again I'm done, now he lied to me saying he took some adderall which he isnt prescribed and I knew he was lying by the way he was acting. He knows how I feel about him watching p**n it makes me feel 2and place like I'm not good enough and I came home to him all messed up and a ton of p**n on his phone and when I asked him about it he looked me straight in my eyes and lied and then starting bringing up old shit in our relationship and we argued. I dont want to go through this again how do I tell him I'm done ?

“I’m done. I told you I would no longer be with you if you continue doing drugs, & you watched porn knowing how I feel about it.” & ask him to leave, or leave his place.