You are certainly not the first monogamous person to get involved with a poly man and wonder. Some realise that they are polyamorous, some feel that they are monogamous and happy with that one person, while allowing that other person to still be with others.
As long as you focus on what you are feeling, and not about labels, you'll do fine :-)
I hope you find the resources that will help you on this forum.
"Listen, or your tongue will make you deaf." - Native American Proverb