He's 19, I'm 20.

So I was in a closed monogamous relationship at first with this guy we have been together for 6 years and currently engaged. He started to discuss with me about the possibility of me being open in the relationship while he stays monogamous to me. I will say that I've had issues in the past remaining faithful to him I have previously had another relationship for 3 months while staying with him as well. I believe that may play a part in how he feels. I worry that this means he wants me to ultimately leave him. We've been on and off fighting for a year now and him bringing this up now seems like him trying to get me to leave him without him leaving me because he knows my mental state....or I'm just overthinking everything like I normally do.

Regardless I am still a bit unsure of if I want to open the relationship on my side the times in which I have been unfaithful were mostly because I was driven to do so by him he hasn't been the nicest person in our relationship because he's very emotionally unintelligent while I'm an emotional thinker. He also puts things in a harsh manner without meaning to. He follows the philosophy of Friedrich Nietzsche if that gives you any insight to the type of peron he tends to be, But I want to make him happy and he's been expressing lately that he is incredibly unhappy in our relationship and there's something that needs to change or else were going to break up and I don't want that to be the case.

To be honest I guess I'm just asking about other peoples opinions on this situation, is it a weird thing for him to ask? Should I try and do it for him? Dose anyone else thing I'm reading too much into the situation? What are yalls experiences on situations similar and how did y'all handle it?