So sorry you find yourself here, but you are in the right place and will get great advice. Some of it may be hard to take, but everyone has YOUR best interest in mind.
First off, you definitely have a WW. Any time an OM is involved, it means wayward. She has followed the standard WW script so far in terms of the emotional distance, trickle truth about OM, and the seemingly instant turning cold after BD, like she just flipped a switch.
The thing is, your W has been involved with this OM for quite some time and has been thinking about leaving for a while. She detached from you and your M months, or even years ago. Yes, it's terrible that she didn't have enough consideration to talk to you about it, but welcome to the club. There are many LBH's here, including myself.
First rule - don't believe a word she says. The EA is more than likely a PA. Also, I highly doubt she has cut off contact with OM at all. She's probably in daily contact, but has learned to hide it better from you.
I suggest you read through all the homework that Cadet posted above, but start with Sandi's threads. The first is "A list of dos and don'ts for the LBS (left behind spouse)" and the other is "For Newcomer LBH with a Wayward Wife by sandi2". Both are extremely helpful and contain a wealth of information about the WW mindset.
You need to immediately set some firm boundaries. Your WW will try every trick in the book to emotionally manipulate you into giving her what she wants. It's hard to hear, but she does not love you and cannot be trusted at this time. You probably won't believe me, but please trust that I know what I'm talking about. I let my WW treat me like a doormat for months before finally standing up for myself. Had I been more decisive early on, I think I could have totally changed my situation, and possibly avoided D. This early time after BD is critical. Please read the homework and define some clear boundaries soon.
You should separate your finances ASAP, including bank accounts, credit cards, etc. There are LBH's who have been financially wiped out by a WW in just a couple of weeks - do not trust her. I would also set up an initial consult with an attorney soon. You don't have to file for D, but find out your options and get familiar with the process. Knowledge is power. Avoid contact as much as possible, and never initiate. If you don't have kids together, I would probably suggest NC at all, until she has left OM and is willing to discuss reconciliation.
I know how overwhelming this all seems and you are reeling right now. Others will be along shortly with more advice. Do your best to follow it and take action to protect yourself. Hang in there, it's going to be a VERY bumpy road.
Me 47 W 42 T 24 yrs M 18 yrs W living with OM BD1: 3/7/2015 (A with OM#1) BD2: 4/11/2015 (A with OM#2, W moves out) WW filed for D, papers received 9/18/2015. Meeting to determine child custody 9/29/2015.