My first boyfriend has changed- should I leave?

I feel so lost on what to do please help me and my bf are both 18, been with each other for over 4 years. And he’s now telling me he wants to do drugs when he goes to a festival this year and he has told me he wishes he could do things single people do. I sometimes feel like cause I’m his first proper relationship he doesn’t know exactly what to do in some situations. He has been slightly unfaithful in the past and flirted with girls on social media and he has kissed a girl before. I don’t know wether to stay with him and accept he needs to do some things and wants to experience it or to leave because his morals are becoming so different to mines. I have sometimes wondered if cause it’s the first relationship it wouldn’t last but I’ve always thought we have but now he’s saying things like that it makes me so upset. Just the thought of him with another girl makes me hurt so much. I’ve got rid of majority of my pals because they were fake and stuff so I wouldn’t have girls to be there when it happened to meet up with(

I feel so lost on what to do please help me and my bf are both 18, been with each other for over 4 years. And he’s now telling me he wants to do drugs when he goes to a festival this year and he has told me he wishes he could do things single people do. I sometimes feel like cause I’m his first proper relationship he doesn’t know exactly what to do in some situations. He has been slightly unfaithful in the past and flirted with girls on social media and he has kissed a girl before. I don’t know wether to stay with him and accept he needs to do some things and wants to experience it or to leave because his morals are becoming so different to mines. I have sometimes wondered if cause it’s the first relationship it wouldn’t last but I’ve always thought we have but now he’s saying things like that it makes me so upset. Just the thought of him with another girl makes me hurt so much. I’ve got rid of majority of my pals because they were fake and stuff so I wouldn’t have girls to be there when it happened to meet up with(