Jealousy tends to be the word we use to describe fear of loss. Sometimes that fear can be projected on to another person, the "rival", or can cause controlling behaviour towards a partner (domestic abuse). I'm sure you don't want it to get like that so good on you for reaching out for help.

Kevin has an excellent compilation of links that I can't just copy and paste as I'm on my phone. But you could also try the search bar.

Bottom line is, it's not her seeing anyone else that would end your relationship...that's the point of polyamory. Maintaining multiple relationships. What would (end your relationship), is that your current has reached its own stopping point, regardlessof anyone else. Not everyone is supposed to be together forever, and if you end up needing to leave so you can find your next totally monogamous next partner, then you have that freedom, too.

But since you are currently wanting to maintain your relationship and manage the jealousy, please do browse the links, and perhaps buy yourself The Jealousy Workbook. Google it or find a link in our pinned resources in Golden Nuggets. Sorry for the lack of links, I'm on my phone and it's just a bit difficult right now.