Search on the forum for 'mono/poly'. It sounds like to me that you are not so into poly, maybe not really into swinging or open relationships, at least at this point. But your concerns, fears and other emotions are very common. Lots of people feel like they are not enough for a partner exploring poly or open relationships. Lots of people feel jealousy or envy that their partner has found another person and they have had no luck so far. It can be worked out but it will take a willingness on your part and hers to really look into your fears and emotions, understand where they come from and address them. It's hard but people have done it.