["I have a small party at my house last night and didn't invite my best friend.", 'Oh, really?  Was something wrong?', "No, I just though she wouln't get along with the other people I had, but I still feel bad I didn't even tell her about the party."]
I can understand your feelings; but I suspect you made the right decision.  Maybe it is something you can talk with your friend about.  A good friend will understand.
I have a small party at my house last night and didn't invite my best friend.  Now I feel bad about it.