Hi Jp - and welcome,
Korindino sums it up pretty nicely. It's a lesson in how we are raised, what beliefs we inherit etc. Now is a great time to take some time to dig deep, look back at WHERE your particular beliefs and understanding come from. And remember - you are an individual, with free will, and have the right & responsibility to chose the life you feel will bring out the best in yourself , and people who your life touches.
So you see - there are two components to this.
Yourself - and the other person(s).
A big part of any good relationship is a shared desire for the other person's happiness. It's about respecting them and their desires in life. It's not about competition, a battle of ideals, winning,losing etc. And as Kori points out NOT about OWNING !
Then there's yourself.
Nothing about being in a polyamorous relationship requires you to be polyamorous yourself. But it DOES require that respect and understanding due another human being - especially one you care for. Your partner will no doubt understand and respect that.
All of us are different in what a relationship means to us and how we flow in it. We have at least one individual here on the board that is mono minded but basking nicely in the heart of a poly relationship. It's ok. More than ok - it can be beautiful.
But it will require really doing some deep digging into how you view the world and your place in it in regards to love & relationships. Some change is likely to occur. But the knowledge gained by going through the process can have huge benefits in many areas of our lives.
Good luck, keep us posted. Lots of good people here more than happy to lend an ear - or a hand/heart to help you travel this road.