I violated academic integrity unknowingly by sharing assignment resources with fellow student. What should I do?
You are overthinking this.  I don't want to make light of academic integrity, but given the cheating that does happen all the time, this is a very minor offense.  We all make mistakes, and what you need to do now is learn from your mistake (one of the things you need to learn is to not freak out about making a mistake).  The appropriate punishment is what you're experiencing right now. Going to the professor now would be a selfish move that is intended to give you peace of mind at the expense of drama and stress for your fellow student and the professor, so don't do it.