Why is being involved in what she has with someone else so important to you? Do you think it would give you some sense of control, or that you are protecting her? Is there some mistrust you have? Do you feel like she invaded your online fun and took it to an extreme you weren't ready for?
If I were you, I would keep asking myself questions to drill down and understand the underlying belief system and feelings you have that feed into your hurt over this. Remember -- she didn't hurt you. You feel hurt. Take responsibility for that, and don't give in to telling yourself you are a victim. You went into this situation willingly. Feeling hurt is usually a response to a thought process that is deeply ingrained in us. You don't have to feel hurt, but in order to extricate yourself from the discomfort of the feelings you have, you need to keep up with the self-examination, look at old beliefs that no longer serve you, see if it brings up familiar hurts from the past, get to understand how your mind works. And keep talking, talking, talking with her.
I also wonder why you want to get your relationship back to where it was. You can't unring a bell. It will never be the same. Never. Life is not static, we are all always moving, changing, growing. The sooner that you accept this, the more at peace you will be. Look, you can choose to be miserable and hurt or you can choose to see your life as a new adventure and the change on the dynamic of your relationship as the next exciting step on the path.