We all have dealt with jealousy of some degree or another. Well, there is the occasional person here who claims to have never felt it. But then there is envy as well, which is similar.
Do a tag search here for "jealousy" and "poly-mono."
Generally jealousy is based on fear of loss.
Just because your new interest wants to have multiple partners, does not mean you need to. You might, someday, be interested in doing that. There is no hurry. Or you may wish to remain mono, even if your bf wants multiple partners. There is no one right way to do poly.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37