I violated academic integrity unknowingly by sharing assignment resources with fellow student. What should I do? So, before I tell my story, I'll qualify by saying I have a bit of a guilt complex and its possible I'm overthinking all of this.  

I was looking through my university's honor code and I came upon "collusion," in which students are forbidden to work together on assignments unless explicitly authorized.  This caused me to realize that I had probably broken this rule last semester on a paper in one of my classes.  The class is over, I got an A on the paper and an A- in the class.

To be clear, I didn't see what I was doing as bad at the time.  We deliberately avoided sharing outlines with one another, and neither of us saw each other's paper.  What we did do, however, was work together for a few hours and compile a single google drive file of "paper ideas" in which we assembled references in the two texts (the paper was a comparison of two philosophers).  Neither of us had completely read both authors, so list of references and ideas was helpful.  Again, this wasn't an outline, just a long page of notes related to the authors, from which we separated and drew our own outlines and wrote our own papers.  

I know ignorance is no excuse, and I feel pretty bad about this situation.  I'm wanting to go to my professor and explain the mistake, but I'm concerned about it because it was an honest mistake and I'm not sure I want to drag this other person into it.  She is graduating in two weeks while I have another year to go.  

I'm just trying to get some feedback as to what I should ethically do as well as potential outcomes of this scenario.  I certainly recognize that I shouldn't do what I did again.
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.