Polyamory

I'd like to start by saying, from you post, it sounds like you're handling this fairly well and doing a great job about communicating your need. While it makes sense that you'd want to try polyamory at the same time you're redefining your relationship with the person you were married to, both of the things you're trying to do put immense strain on the relationship. Individually, both things are going to be highly emotional and require a lot of clear communication.

Polyamory is like sex, if it isn't a "fuck yes" its a "no". I don't necessarily want want to say its hard, but it is a lot of work. Its a lot of talking about emotions, trying to coherently share how you feel about things that you may have never verbalized before, trying to understand other people's experiences, and re-learning how relationships can be defined. Your partner may be scared but he may also know that it isn't for him.

When talking about this new guy, is he polyamorous? That could stop the whole process right there. Thinking of that, and maybe the kids are calling it something different these days, look into NRE or New Relationship Energy. Many people have cheated or tried opening their relationships because they wanted a specific somebody else and when it didn't turn out how they wanted they found themselves single. If this new person wasn't in the picture, would you still be interested in polyamory? If your current partner started dating someone else and you were only dating him, how would you feel about it?

There is a lot going on, while I don't know what your relationships look like, in your shoes I would either rebuild things with my current partner and look into maybe opening the relationship a few years into the future or pursue the new guy and start things off in an open relationship. Whatever happens, I hope that all of you end up with the best possible outcomes.