Polyamory

My long-time girlfriend and I started in a poly relationship, but have been monogamous for the last six years. She has expressed interest in dating another man, and I gave consent without too much thought. I mean, we did it before. What could have changed? Well, she found a guy she likes and they have gone on a few dates and such. We recently had one of those hard talks, because she was pushing for me to meet him, because she thinks I need more friends, and "we have so much in common," despite me adamantly refusing to. Eventually I was able to convey what the thought of meeting him did to me, like raise feeling of jealousy, fear of being left alone... stupid stuff. Now even though I don't have to ever see him, there is a burning feeling in my gut. Is it jealousy? I don't know. But... how do I know for sure I am ready for an open relationship? Should I ask her to call it off with her metamour?