Hello, Wanda. Welcome to SI.
Your "years later" story sounds a great deal like mine, but I am the WS. My BH (boyfriend at the time) and I were college students in an LDR when I developed a crush on the guy across the hall from me. Over the course of several months, I broke boundary after boundary and eventually developed a full blown affair. I finally ended things with the OM and realized what a terrible mess I had made, and I was tempted to hide it. However, this was at the height of the AIDS crisis in the US, so I realized I had to come clean to my BF because of the health implications.
The next time I saw my BF, I confessed to the sex and "I love yous," mostly expecting him to break up with me. I was prepared for anger, but not devastation. I immediately started trying to limit the damage. I decided that BF needed to know about the sex because of HIV, but he didn't "need to know" that it was a planned event on a weekend getaway. He didn't "need to know" that I spent almost every night in the OM's room, or that OM actually tried to propose to me in the last days before the end of the A. I convinced myself that these were irrelevant details because I had given him the highlight reel. The decision in front of him was whether he wanted to break up with me for having sex and love for another man. He knew that I had, so I justified hiding the rest.
It sounds like you have a deep understanding of how cruel and flawed this logic was. It's not possible for information to be simultaneously so inconsequential that it doesn't need to be disclosed and also so damaging that it could end the relationship. However, WS are excellent at that kind of pretzel logic. Meanwhile, my BF knew that OM wanted nothing more than for BF to put a foot wrong and drive me back into his arms. I was apologetic and supportive for a few months, and then I started to run out of patience, asking if there was any point in tormenting each other if he was never going to forgive me. BH panicked and rugswept, and a few months later, we got engaged.
There are many other details to how this evolved that I'm leaving out (the old timers here know what they are) so I can stick to the part that is most relevant to you. My BH did not recover as the years passed by. We got married, had kids, and built a life together, but all that unprocessed pain was brewing under the surface. It became particularly strong at times when he was struggling with something else in his life, like issues with his father or a period of job loss. Anything that prodded at feelings of loss, insecurity, or inadequacy ripped that wound open again. I was unaware of this. I knew he had periods of depression and anxiety, but I thought the A was in the rear view mirror as the decades stacked up.
In 2018, my BH decided that he couldn't hold it in any longer and started asking me questions about things that had never added up. He knew me well enough, especially after 30 years together, to know that I was not the type to suddenly fall in bed with someone else. There had to be more to that long ago story that I was leaving out. I panicked. I had spent those years burying the truth, trying to hide it from myself as much as him. The girl who prepared herself to be dumped at the time of the original confession was now a middle aged woman with enmeshed finances and three kids. Telling the truth felt suicidal, and so I doubled down on the lies.
I can't tell you what your WW is thinking, but I can tell you these things from my experience with my husband. You need and deserve all of the truth. Your pain is valid. If I had come out with the truth -- every last bit of it -- 35 years ago, then my husband would have had the agency to make an informed decision about marrying me. He might still have ended up regretting it, because it's impossible to know how the journey will end when you're just starting out. But until I finally let go of the outcome and told him everything, 8 weeks after he started asking in 2018, healing simply was not going to be possible.
I hope that you learn from the collective wisdom of SI and get the support you need. The most important thing my BH learned was to forgive himself for his pain. It wasn't water under the bridge, it wasn't irrelevant, it wasn't any of the self-blaming things he told himself about how long it had been or how young we were or how he should be over it. It was a chronically infected wound that needed to be cleaned out. Now he has a scar, but a scar is far easier to live with.