Navigating an Open Relationship with a Partner Who is Uncomfortable

I ask my partner every so often his stance on being open or having fwb, but he gets in a bad headspace everytime. I told him when we first got together that I was interested in an open relationship, but I feel like it's never going to happen. I'm interested in the entire romantic and sexual aspect of our current relationship, I just wish I could explore more but he gets so mad when I bring it up that it's just an inconvenience or it makes him uncomfortable but I feel like I can't stop wanting to explore things with other people, but also maintain a healthy loving relationship with him.