Kudos for coming clean! If you kept that bottled up inside it was going to be the doom of the relationship anyway, one way or another, and would've eventually grown into some nasty emotional cancer for you. I think it's always best to air these things and let the chips fall where they may - others may disagree.

I am so sorry that a seemingly happy, positive relationship has ended on such a note - but based on her responses after the truth came out, I suspect that "the other guy" would have eventually become an issue in your relationship even if you hadn't even read those emails.

Ultimately, there are some good things to come out of this:
1. You have learned a valuable lesson about reading your significant other's private email - unless you have prior reason to suspect infidelity or dishonesty and are just looking for ammo, then it is always best to let private things be private, and if you need to peer inside that world it should be with her consent and preferably after great soul-searching as to "why" you feel the need to go there.
2. You have forced her to face something that does not (to me anyway) seem like a healthy facet of her life, nor something she seems particularly willing to explore in terms of the "whys" and the potential repercussions. Everybody has skeletons in their closet of one size and shape or another, and generally they are harmless. But when you have a skeleton in their and you don't know why, and you can't explain why you continue to keep it in there, I think that there are deeper, related issues that need to be explored and addressed or that skeleton has the potential to become an infectious agent, something that will get bigger, uglier and more damaging over time. It is good that she is being forced to evaluate that relationship and what it means to her - right now she's pissed about the invasion of privacy, probably embarrassed and clearly confused. You are the one who is getting the brunt of that, not only because you instigated it, but because you are an easy target for other things going on in her head that she is unable to process.

My hope is that she can evaluate both relationships, figure out what exactly the other guy means to her and why, come clean to you with all of that, and the resulting release of all of that tension and confusion will make it possible for both of you to evaluate *your* relationship on its merits, with both of your serious transgressions out in the open and decide if you have a future with everything actually out on the table this time.

Good luck man, I had knots in my stomach reading your initial post. It sucks to screw up what seems like a relationship with "the one" by being nosey, but let's face it - there were clearly issues on both sides (her not being totally honest with you or even herself about this other guy, and for you...i think there are trust and control issues...not to some abusive level, but the seeds are definitely there) that if not addressed would have eventually raised their ugly heads in one way or another. Better to go through the pain of exposing these things at 6-months in rather than having them slowly erode things after you're married and have kids, right? And at this point, if there really WAS something there, you have the opportunity to salvage it if you can both put this incident and all it represents in its' place.