"I don't think there's any authentic goodness in me"
I think there is a purpose as to why you are writing this post and in this forum. I think this way you are, which I don't doubt, is not necessarily how you want to be.
I would also venture to guess that you're not particularly satisfied with yourself, or your life.
Just your simple indignation at the notion of helping the other, is indicative of a strong sense of idealism--what should and should not be, what is right and wrong. The indignation response will indicate a low ego strength as well (low self confidence). Your description of your behavior will also have poor results in relationships--both friendships and personal ones--and that further backs up the points above.
I believe you; you probably are a person that is selfish and concerned primarily with yourself and self interests. But you've been trained that way--how else are you supposed to know how to behave if that's how you've been raised by your parents? I'd probably be that way if I were in your circumstance, and so would others.
I would guess that the inauthenticity of your interactions is isolating to you, unconsciously, and manifests as a certain persistent discontent with a lot of things in life.
You say you 'feel guilty'. I don't think that's accurate. I think you feel like something is wrong, but you can't identify what it is, and you're calling it guilt.
Change happens by action though--in my own life, I try to always be better than I have been in the past, always try to do better, be better, and improve my life.
My advice to you, would be the next time you are in starbucks, specifically go in with the plan to pay for the person in front of you. Not necessarily to become a better person--but just to see what happens to you, and how you feel afterwards. Maybe good, maybe bad, maybe nothing.
I say do that, and write here once you have.