Opening up a monogamous relationship?

I've been with my partner for three years and am still very much in love with him. I've never wanted to be in a monogamous relationship longterm. I was clear about that at the beginning of our relationship and he seemed OK enough with it to be involved with me, but still very insecure about the idea. I brought it up recently because I trust the foundation of our relationship and our ability to work through issues. He was fairly unresponsive and wants to postpone talking about it again. Being nonmonogamous doesn't seem to be something he can be comfortable with. I'm not entirely comfortable with monogamy and here we are a few years later with a fundamental difference in what we look for in relationships. I don't want to put pressure on him to be someone other than the person I fell in love with, and I also don't want to sacrifice what feels healthy for me. Compounding this issue is his low sex drive and my feeling lonely physically. Anyone been there and had success in navigating this situation?