I very much agree with laughing owl.
Just over a year ago I was in a similar situation to your girlfriend, I wasn't sure about having a poly relationship at all - in fact it really scared me - and sometimes I'd get lots of irrational or resentfult thoughts like 'I'm putting in this effort and making a sacrifice, why shouldn't he?'
Which was obviously the insecurity talking and whilst I'm not always entirely comfortable now, in many ways I've come to really enjoy being in a poly relationship.
Laughing owl points out an excellent way to help get rid of any resentment that might build up in a partner. It definatley worked for me. The more my partner showed me that I was loved and appreciated no matter whether he was seeing someone else or not, the more comfortable I became.
Hopefully, if she feels loved and secure, she will come to see the merits of being in a poly relationship aswell