Welcome to our forum.
There is a lot of information and good people to talk to here, so dig right in. Perhaps start with our Golden Nuggets board
and go from there.
It is probably a good idea to take a "break from poly" while you get things figured out between the two of you. On this site, you might consider a search (or better yet a tag search) for "communication" as that is probably the #1 key to making a poly relationship work. And there's never a shortage of new things you can learn about better communication.
All kinds of challenges can arise in open/poly relationships. Sometimes NRE (New Relationship Energy) causes us to "lose our head," overlook flashing red signals in our new love interest, and neglect and ignore our original partner without even realizing it. Perhaps that's what happened to your girlfriend. I'd have to know more details to be sure.
Hope Polyamory.com helps you get your stuff figured out so you can start your poly life again.