Hi James,
I'm sorry to hear about your wife leaving you for another guy after 12 years.
Personally, if my husband left me for another woman, I would wait on God. Yes what your wife did is terrible, it's horrible, it's sinful, she broke her marriage vows, etc.
I believe that God calls us to forgive. God is all about restoration. He forgave us for a boatload and sins.
Yes you have grounds for divorce, but I would like to encourage you to wait upon God and see if He'll work a miracle in your marriage. I have heard where the injured party has prayed, waited upon the Lord, changed their own sins that might have attributed to the break up of the marriage and God worked mightily in their lives.
In one case, the husband waited over 2 years for his wife to change; God worked in his wife a change of heart and they both had a better marriage in the end. Not all will change, but it's worth it to wait because our God is a miracle working God. There is always hope if one is willing to forgive.
I would recommend finding a male prayer partner and a confidant so you can unload all you're going through. They can help you through it all and help you see things more clearly if they're objective.
I want to say kudos to you for loving your wife through thick and thin, for better and for worse. I know I would still love my husband if he cheated on me. I may get angry, hurt or whatever, but I would still love him.