Amazing how an old event can cause intense fresh pain.
Actually for you, it just happened 5 minutes ago. As far as your brain is concerned, it happened one second ago.
How long had you been married when the adultery happened?
Part of the pain you feel and will feel is that she has essentially been married to you on false pretenses for most of your marriage.
This has poisoned your marriage in ways profound and minor, in ways you will slowly realize.
Not long ago, I was part of a discussion encouraging a woman to confess her short term affair with a neighbor from 8 years ago to her husband.
They had moved away from where it happened. She had managed to keep it a secret.
As the discussion went on, it became clear how the secret had affected her marriage. She would feel guilty about it every few months or so and would experience depression and would withdraw from her husband for a few days.
We tried to help her see that this behavior has made her marriage less than what it could have been -- a lot less. Her husband obviously sensed something was wrong; he just didn't know what it was.
If you think back, I'm sure you will begin to realize various ways and various times in your marriage that were likely impacted in this way.
"The truth will out" is an old saying for a reason. I'd be curious about her real motive for confessing this now. There's always more to the story. It's doubtful she's giving you the "whole truth" and is really giving you a managed narrative she controls.
Just reading between the lines, it doesn't seem she's taking this as seriously as you are and is interested in minimizing and rugsweeping it now that she has told you the managed narrative she wants to control.
I think the advice to treat this as a new revelation for a new affair is a good one. That means the following:
1. See an attorney first and foremost so that you can understand your options. You may not want to divorce at all, but it would be good to know what divorce actually looks like in your state.
2. Put a VAR in her car. This is so you will know how she may be discussing her disclosure with friends and family. It also may give you information about WHY she decided to tell you now. Tell no one about your VAR, not even your closest confidantes.
3. Ask her to turn over her cell phone for you to run retrieval software on it. Since the affair is allegedly far in the past, this is really more about seeing what her reaction is to this request for transparency, and also about communicating to her the gravity of the situation and your shattered trust.
4. Ask for log on information to all of her social media, email or other similar accounts.
5. Ask her for a written detailed timeline of the adultery. A true accounting without trickle truth, red herrings, omissions, etc. Since she's already disclosed to you the basic outlines, she should be able to provide this within a week. It should not be a perfunctory one page document. It should be detailed. Putting this in writing also drives home the toxic and ugly nature of her actions.
6. You then ask folo up questions after reading this timeline based on your pace and your schedule. She should not respond to your questions with "I don't know" or "I don't remember" or "I've already told you everything."
7. Schedule a Polygraph examination. Ask no more than three precise questions to test the veracity of her story and the timeline. Cost is about $500, takes about two hours' time. Drive her to the appointment.
NOTE: one of the questions should laser in on whether she has had any other adulterous activity of any kind before or after this affair. The polygraph examiner can help word the questions.
8. I'm unsure whether a "no contact" document is needed after this much time, but you might consider having her write a letter to her affair partner/partners detailing the tragic severing of trust that has occurred.
9. At the same time or before this happens, tell the other betrayed spouse or spouses. This is practically an ethical obligation now that you know for the same reasons you would want them to tell you if they were in possession of this same information.
10. Tell her family, your in laws and her siblings. They need to know what has happened and what you are going through. They will support their daughter/sister and likely circle the wagons around her over time, but this shatters any illusions she might have that this is "no big deal" after so long a time. It also prevents her from spinning a false narrative about "marital troubles" with her family.
11. Immediate IC for both of you, preferably with a betrayal trauma specialist for you both individually. Betrayal trauma specialists can also help manage the additional disclosure process.
12. Implement a 180 on her. Read about it more on this site in the sidebar. You might also consider a temporary/therapeutic separation so you can be away from the source of your pain. We wouldn't send a shell shocked soldier back to the front lines. You should not have to be triggered by your WW day after day right now, unless you think that you want to be around her.
13. Do not accept the following: continued trickle truth, blameshifting, attempts to rugsweep, attempts by her to rewrite the history of your marriage, minimizing her behavior, attempt to DARVO (deny attack reverse victim offender).
[This message edited by Thumos at 5:19 PM, October 14th (Wednesday)]