First things first, do your homework that Cadet sent you. It's critical.
Second, this isn't going to resolve itself quickly. It's going to take a long time. I know that's not what you want to hear but I'm going to shoot straight with you. Fasten your seatbelt.
Your situation is not hopeless. Many people have come back from the brink of divorce, some even remarried after divorce.
Most important, you need to stay CALM. You will feel fear and despair that you never knew possible. This is normal.
You are among an awesome group of people that are or have experienced the exact same thing you are. I consider most of them family.
Come here and post often, we will offer our best advice and listen to you vent. If you feel you are going to react or say something to your W, come here first before you do anything. We will talk you off the ledge and help prevent anymore damage.
Your W is NOT the girl you married at the moment. She's having an affair and is in fantasy land. Consider her an addict looking for her next fix. She will do and say things you didnt know were possible.
Take care of you. Get in the gym, meditate, run, read, pray. You're going to need it.
Lastly, the journey of a thousand miles begins with a single step.