I was in a monogamous married relationship for over 20 years and never considered poly life — until we did, and now we do. So, things can and do change, but I’m also agreeing with Joe and Rose and others that it’s perhaps not for everyone and it’s good to know yourself.
And, I also believe that there’s a lot of cultural narrative that supports jealousy as inevitable and a sign of true love. I even heard one woman state that she thought that the absence of jealousy in her relationship would signal indifference. This is an erroneous and non-serving idea that comes straight out of patriarchal norms. The other piece that is absolutely potentially changeable is insecurity. The more secure you are in yourself, in your own worth, desirability, what have you, the less jealousy is a factor. Working on that can and will diminish jealousy, even if you always stay monogamous.
But I also think it’s easier to be non-jealous in polyamory. You don’t have all your eggs in one basket, so to speak, so I think there’s less of an inclination to defend against “invaders” because it’s not an all or nothing proposition as it is with monogamy.