There were times that Runic Wolf felt that he was losing me to NRE along the way and whether it was the impression I was giving or his own personal issues didn't seem to matter. What mattered was that he told me and gave me the opportunity to reassure him and try to be a better me.
Your girlfriend may truly want to believe that she can give up this girl for you, she may want with all her heart for you to know just how much she loves you, but she also might have a bit of New Relationship Energy of her own or be developing feelings for her girlfriend that cloud her ability to do that. So maybe giving her up isn't an option at the moment, but allowing yourself to present your side to the girlfriend, explain that you're struggling with having to share her because for 2 years and 5 months she's been all yours and you need some time to adjust and some reassurance that she isn't trying to steal your girlfriend from you. Having that assurance from her could mean a world of difference in how you interact with her (which benefits her), how you see the situation and act with your girlfriend (which alleviates the stress on your girlfriend, which benefits all 3 of you), and how you determine what you yourself want/need/ and can handle from this now poly relationship.