Sorry that you find yourself here, friend.
You are regretfully now on an emotional ride that you never asked to be on...and it can be quite brutal. But here is the one thing, the most important thing, that you need to not only understand, but believe through and through---her cheating is 100% her fault. Period. Do not in any way, shape, or form, accept any blame(condition of the marriage, your lack of attention, etc.) for her actions. This is all on her, and if she isn't willing to accept and understand that, then you will never return to a healthy marriage.
It really will boil down to (2) conditions to save the marriage: First, do you even want to attempt to save it? And second, is she truly remorseful?
The answer to the second question is easy---she is NOT remorseful. Not even a little bit.....YET. I emphasize the "yet", because it is rare that waywards are instantly remorseful. Yes, it does happen, but not that often. And the reasons are many, the most common being that they have allowed their boundaries and coping mechanisms to become so skewed, that their mindset is basically rewired. They very often have rewritten the marriage, lost respect for you, and often even despise you. It sounds ridiculous, but that is often EXACTLY what they have done....because how could they justify cheating if you and the marriage were great?
As to the my first question---whether you even want to attempt reconciliation---that may take a little time for you to answer clearly. Just like your wife's head is currently screwed up, so is yours...and it takes time for the immediate emotions to settle down. In my opinion, best way to decide if you want to reconcile or divorce, is when you get yourself to a mental place when fear is no longer a factor of staying or going. It is then that your CHOICE will be a clear and healthy one. So even is your mind and heart are screaming "STAY", it would be best if you didn't feel dependent upon that urge.
The goal here is for you to survive infidelity, and often that means divorce. And for the record, I am all for reconciliation, but only if it is a healthy, committed effort by both parties.
So take time to ask yourself what you really want. Besides the obvious, like having your "old" wife back, what do you really want? After you decide, and it may take some time, then don't be afraid to act.
You claim that you knew something was wrong years ago, and it hasn't stopped nagging you. That is what infidelity does---it stays planted in our minds, and until we learn to deal with it in a healthy manner, it will continue to spread, like a cancer. It has to be targeted, and acutely dealt with, to stop the toxic spread. And it often hurts to do so, but the pain is far less in the long run, when dealing with it head-on. Don't be afraid to push for the truth....to demand such. Don't compromise your integrity for the fear of a failed marriage. Please read in The Healing Library(yellow box in top left corner). Read the threads in this forum with the bullseye icons...they are normally on the first few pages. Read up on the 180....a detachment tool that helps you get some emotional clarity for making better judgements.....even though it seems counterintuitive. Read, post, read, and post some more---the more that we know, the more that we can help.
You are going to make it through this, but like I said, it isn't easy....obviously. But you will emerge with a stronger will and resolve, because that is what is going to be focused on.
Good Luck going forward, friend.