Impressions from the past can definitely do a number on you in the present. Jealousy almost always does boil down to insecurity or fear, and your history with your ex-wife would certainly seem to explain why you're feeling jealous about your current partner having others.
If you do choose to meet her other partner, I hope it works to help you get past the jealousy. Because "just meet your metamours, then you won't feel that way" is quite common advice among poly people, I do want to put in that for some of us, meeting the metamours makes the insecurity and fear *worse*, not better.
Personally, if I don't know a metamour, my brain doesn't have a lot to play with in terms of my anxiety and depression creating worst-case scenarios. All I know is there's this woman named Whatever who my partner's seeing, and there are still fears and insecurities, but there's little to hang them on. But if I meet a metamour, now I have a visual image of her. I know what she looks like, and my brain has probably informed me that she's way prettier/thinner/sexier/whatever than I am. Mental images of her with my partner intrude no matter what I do to fend them off, though I do shut them down as much as I'm able.
I think for the majority of people, meeting their metamours does help, but I also think it's important to note that that doesn't work out well for everyone.