It sounds like theres a serious lack of self esteem on your behalf here. If you have not been together that long then maybe you just need to adjust to the relationship. When you care about someone and feelings develop with the progression of the relationship, you feel happy that that person has the space to enjoy herself, even if that doesnt involve you in some way. You are not soley the only thing that is happening in her life; she will have loads of fragments that will all balance out ; her relationship, her friends, her family, career, etc.

Maybe theres a deeper personal issue with you that has come to light with you alone with your thoughts and her not there to occupy them? (i might be wrong, its just a thought)

Hope you feel better soon. hugs x