Well, you would have a couple of options.
1. Decide to stay monogamous so you can be with that person. Possible pitfalls include feeling resentment, restricted, suppressing your emotions etc
2. You can break up with said person on the basis that you want two different things. This way you would be free to be polyamorous but you would have to end the relationship with that person.
3. You and that person could try to make a poly/mono relationship work and there are many discussions, blogs and people on here that have lots of experience with that. Mono, Sage and Vodkafan for starters. (I'm sure I'm forgetting someone!)
Plenty of people (including me) never went looking for poly, it just fell into their laps. You must consider if you need to be poly in order to be happy. If it turns out that being with that mono person is so amazing and you're kind of ambivalent about poly, and you know that staying with that person is going to be fulfilling to you....then maybe you would choose to stay. But consider if you love that person dearly but now that you feel poly, you don't think you can happily be monogamous, then maybe you have to move on. Ultimately, it's about what decision will best suit your needs long term.