Gotcha. It's normal to feel what you're feeling. Lots of people seem to be ok with an open relationship in theory but then still feel pain, betrayal, jealousy, what have you when they first have to actually deal with it. It's something you work on, with your partner's help. It seems like for many it's a phase that passes with time as long as their partner sticks to agreements and is sensitive and supportive. For a few, unfortunately, it doesn't seem to go away and they end up realizing they can't do open/poly.
Good resources here, under the Jealousy & Insecurity tab on the sidebar: http://www.morethantwo.com/
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.