He slept with someone else

My boyfriend moved in with his friend and we hardly talked and now we talk everyday but he got high one day and fuck another girl i was kinda mad but being high is not an excuse but i still love him and wanna be with him but i feel like he wont believe me saying I'm not mad i told him I wouldn't bring it up anymore and i was sorry to ask about it in the first place and he didn't even want to talk about he felt bad he was sad looking please give me some advice and don't judge me thank you