I know I'm late to the party, but Panda, I really feel for you.
One of the big foundations of poly is open and honest communication. You were not given that. We talk over and over again on this and other fora about the importance of making sure that we don't end up hurting those that we care about due to our ambiguous communications. If she suspected that you had no clue about poly, that it was really up to her to tell you about it, answer your questions. Her not doing that gave you an expectations that this was heading for a regular, monogamous relationship - I can't blame you at all for thinking that. The "pansexual" part is about which genders or gender variants a person is interested in - it has no bearing on how *many* folks at a time.
So whether done through innocent assumption on her part, or a deliberate hiding something - who knows? let's give her the benefit of the doubt on that one.
But the fact is that you want an exclusive relationship, a monogamous relationship. This is a "bottom line" for you - it's not negotiable. Not only do you not want to have relationships with multiple people, you need a commitment from your partner that they want the same. She does not, and it appears that poly is a bottom-line for her.
These are utterly incompatible. No amount of talking, or cajoling or persuading is going to change that. In some ways, expecting that it will is disrespectful of each other.
You two need to recognize that a loving relationship isn't going to be possible because of these incompatible bottom-lines. Don't waste the time and energy trying to convince each other - I've been there and all it does it create resentment.
Accept things for what they are. Find out what sort of relationship (more than likely a friendship) you *can* have, rather than trying to push something that isn't going to happen. That way you can stay in each others lives, and maybe accept *each other* for who you each are.
I understand how hard this must be for you, after you had built these expectations of a relationship with this person. But don't compromise your core values for anybody - you are more important than they are, when it comes to your own happiness and what you truly believe in yourself.
"Listen, or your tongue will make you deaf." - Native American Proverb