Welcome to the club nobody wants to join.
First, your feelings at present are 100% normal. It's often called the roller coaster. You'll vacillate wildly between anger, despair, hate, love, pain, and even moments of peace. I'd suggest that you spend time browsing The Healing Library (yellow box, top left of page). Among other things, look for information on The 180. This is not a tool to punish your WW, nor is it a gambit to get her to act any particular way. Rather, it is a method to create psychological space for yourself, because your main task is to find your heart's truth. This will take time, and many can't find it without implementing the 180.
Just finding out changes your entire life's paradigm. It's like peeling a mask off of your wife and seeing her for the person she really is for the first time. A person who will come home, look you in the eye and lie to you directly, while creating a cocoon of intimacy and love with another man.
The main question you need to answer for yourself is, knowing what you now know about your WW's true character, is she really the kind of person you wish to be married to?
Finding out years later adds its own layer to the shit sandwich your WW has been feeding you. There is the sense of surrealism, as if the last 11-18 years have been a false reality, like The Truman Show or the "upside down" in that science fiction series.
There is the sense of unfairness. As you said, had you known at the time, you would have divorced her. She took your agency from you by lying, tricking you into remaining married to a cheater.
Another unfairness is the sense that she has no consequences. She arrogated to herself the right to decide that her half of the marriage would be a secret one-sided open marriage. She had her fling, her limerent sex, her thrills, while you were being a good engine, bringing home your paycheck, being a husband, even while undergoing a series of major health setbacks. Don't you want that for yourself?
My observation about the "found out years later" scenarios is that, often, the quality of the marriage between the end of the A and Dday matters. Did she realize the gravity of her error after the A ended and throw herself into being the best wife a woman can be? A dream-come-true spouse? You should assume that the marriage between 2009 and 2021 is the best you're going to get. The marriage will never be as good now that you have learned the truth.
By the way, if the boss was married at the time, I'd suggest that you reach out to his wife to discuss what she knows. You should do this without first telling your WW that you will be doing it. Many posters find that the BOW can be a helpful ally in terms of finding the truth, as well as a shoulder to cry on as you share your mutual pain.
If you're thinking that reconciliation (often called "R" here) may be an option for you, I'd recommend reading Joseph's Letter in he Healing Library, and also the book "How to Help Your Spouse Heal from Your Affair". Is your WW cut from the cloth to make the kind of investment into your healing that is described in that book. R takes years of hard work. The WW must carry most of the weight. Even with the effort, it may not succeed. Is your WW a long distance runner? From the initial comments you relate, quite frankly, it sounds like she is not.
but my wife now tells me she refuses to discuss the affair any longer because it happened long ago and it’s too emotionally difficult for her to discuss. WTF! Just like 2004-2009, it’s apparently all about her needs and wants!
I think you know that this is diametrically opposite of what she must do if you are to heal yourself and your marriage. Her approach could not be more wrong. It's called "rug-sweeping" here and I can guarantee you it never works. In fact, if she continues on that path, it will answer your question for you in terms of whether your marriage can be salvaged. Among other things, a strong recommendation for a WW who desires an opportunity to receive (from you) the gift of R is that she prepare a detailed written timeline of the A, including all of the x-rated details. You'll likely find, like most men do, that you cannot R without knowing this (if you choose to D, then you probably won't care).
By the way, though she says that, to her, it happened a long time ago, to you, it happened the moment you found out. I say it that way because the one trait your WW must have to successfully reconcile is empathy for your trauma. Right now, she is showing zero empathy. Empathy by the WW is the cornerstone of successful R. Without it, you can be 100% certain R has no chance.
It is still pretty new in terms of your process. Your WW's default, for over 15 years, has been to lie to you about this. Lying is her normal. Telling the truth is foreign. It often takes wayward spouses a while to get their head out of their ass and see the hellscape reality they have created. I say this because that is what your WW will need to do if your marriage is to have even a breath of a chance.
Perhaps you could begin here by describing how the Dday went. How did you find the dirty poem? What did you do next? How/when did you confront her? What was said? Have you had sex with her since then? Sleeping in separate beds. This place is an anonymous forum that is rich with crowdsourced wisdom about infidelity, all from people who have experienced it first hand.
[This message edited by Butforthegrace at 11:24 AM, April 14th (Wednesday)]