I'm dating someone who's polyamorous, and can't help feeling unhappy about his other partner

Hello! I'm a 16 year old woman dating an 18 year old man, while he is also dating a 25 year old woman. I've been in love with this man for almost 2 years now. We've been technically together for about 8 months, but he has been dating the other partner for 6 months. I'm new with polyamory and I keep finding my self jealous whenever he usually talks about her. He reassures me constantly about how he loves us the same, but I still can't hide the jealousy. The other woman, I'll call her Jane for now, is a very nice and great person! I just can't escape the fact that I don't want them to be together because I'm constantly jealous, the age difference, and so on. He makes me so happy I can't imagine my life without him. But I also don't feel too happy about Jane. I'm very scared to tell him.

It sounds like maybe polyamory just isn’t for you! You like this guy, you even like his other partner, but the arrangement at its core doesn’t feel good. That’s okay! 

This is why I will continue to hold to my point that for some people, monogamy or polyamory is an orientation, something at the core of who they are, and not just a choice they can rationalize themselves into. Everything here is healthy, but it’s just not working for you. 

You may love this guy, and really want to be with him, but sometimes things get in the way of being with people. That’s life, that’s dating. Sometimes you’ll find yourself falling for someone who only dates other vegans, and sees your meat-eating as an irreconcilable difference, but you’re not willing to make such a drastic lifestyle change, and so it doesn’t work out. Sometimes you’ll develop a massive crush on a friend who only dates men. Or who is moving to Alaska next month.  

If this relationship makes you feel jealous and "not too happy,” it’s not the right relationship for you. It sounds like right now, you’d be happiest in a monogamous relationship. It’s awesome that you took the leap to try polyamory - but at your age, the point of dating is mostly data gathering, learning what you like and don’t like, learning who you are in relationships. So this has been a resounding success on that front! You got to sample healthy polyamory, and discovered that it isn’t for you. It’s now on you to act on that new knowledge.