I think my reaction would really depend on the circumstances. Why did I cheat? Why did she cheat? There are so many possibilities. Is this a response to the relationship and the parties not getting what they need from each other? Is this because the relationship is all but over? Is this vengeance? Why did she tell you? What did she tell you? Is this second time with the same guy as before, or someone different? Do you want to fix the relationship, or get out? Do you still love her?
I’m sorry, but the devil is in the details, and if you don’t provide more, I think the answers will be meaningless to you. I think what you are really doing is just asking for sympathy. I’m afraid that most people are more likely to be judgmental than sympathetic (see @uberbatman‘s comment above).
You’re probably in shock. You probably don’t know what to think. As everyone says, a couples counselor might help. Also some introspection might help. Of course you have to talk to each other. If you care about each other and really want to fix it, you will listen to each other without being defensive. If it’s over, you’ll just fight and assign blame and it’ll be miserable.
People make mistakes. It’s usually because we don’t know what to do and we are miserable. We reach out or lash out in the heat of our emotion. As ever, two wrongs don’t make a right.
I think you should sit down are write out your story. First, what happened (for your infidelity). Then the story of your actions when she cheated and just now. I think you should do this for your own understanding, and as a starting point. It might give you some idea of what you are looking for, and a basis on which to start healing or separating. Whether you share the document with us is up to you. If it’s as raw and open as it should be, you’ll not want to share it with us, although if you have great courage, you might.
I wish you peace. I wish you reconciliation, if that’s what you both want. I wish you the grace to identify what you want out of life, and the ability to learn from your mistakes and do better next time. Your relationship (marriage?) is not over, but it is 90% of the way there.