Good news! You're normal! Bad news! You're normal!
The reactions you are having are really common.
It is common to be bummed and unhappy when one partner of a couple trying poly finds another interest while the other partner does not have anyone else yet. Your post read like you were ok with your partner's potential interest while you were exploring things with your friend. Once your friend declined, that seems to be the precipating event in the freakout. Perhaps you are not freaked out about poly/open per se but that your partner has someone and you do not and this generates insecurity, jealousy and other such feelings.
It is also common to agree to try open and/or poly, experience a partner interacting with other partners, try to interact yourself, and find yourself completely owerwhelmed with feelings you did not anticipate in any way. Give yourself a bit of a break. It's a hard thing you are trying on for size. There will be lots of unanticipated bumps and weirdness.
You are experiencing the painful, normal, common growing pains of developing an open/poly relationship. Now open or poly may or may not be for you, or for this relationship with your girlfriend. That's still up in the air. But you are certainly not a disaster or 'poly-handicapped'.