Hi and welcome
There are lots of things your can do to support your girlfriend: check out my blog www.polyamorouspeople.com
, check out some of the earlier posts when I was struggling with the same thing. You can search out threads on here for mono/poly relationships. You can join the livingpolymono group at yahoo groups this is a group for polyamorous people in relationships with people who identify as monogamous. You could also get your girlfriend to join the polymono group at yahoo groups which is for monos in relationships with polyamorous people. Both sites can be found through google.
You seem to be taking this very lightly, even though your post is entitled "emergency". Maybe not, but it sounds like you're surprised at her response. It can be a huge deal for someone who feels monogamous to come to terms with a partner who wants a polyamorous lifestyle. Some never can and you may have to choose between the lifestyle you want and your girlfriend.
Poly/mono relationships that work well seem to be the exception rather than the rule and require bucket loads of work and emotional intelligence. Are you up for it?