I've always been a jealous/possessive person. Through polyamory, those feelings have not only decreased to be replaced by loving compassion and joy, but I've learned valuable lessons in how to deal with my jealousy. It's still here, still poking at me from time to time, but these days I handle it on my own in a healthy manner, or I clearly and responsibly communicate it to my partners.
Polyamory turned out to be the perfect thing to help me grow and heal the wounds I've carried for most of my life. It has changed me irrevocably and for that I am grateful.