I am new to polyamory as well, however am a few years older than you and your boyfriend (26). I only recently figured out that I identify as poly, and it took meeting someone who is poly to put the name to the feelings and thoughts I've had since I was a teenager.
I dont have a lot of advice since I am still very new to this, but I have been doing a ton of reading and research on polyamory and one of the most common themes I've seen besides being open, honest and communicative, is that poly relationships work best when there is a sturdy/stable foundation.
I dont know if age has anything to do with it, except maybe that you haven't had enough time to be sure of yourself and of your relationship. And if you're anything like I was and like many teenage girls are, there are still so many doubts in your head and heart about yourself, your image, and who you are. These I feel could all lead to some of your feelings of jealousy and the jealousy that you're feeling.
I think that if you truly feel that you are poly, that you should work on yourself and your feelings first and working with your boyfriend to create and develop a strong stable relationship before attempting to bring in other loves.
Hope some of this helps or at least gives you some things to think about.