As it was so long ago the default answer to any of my questions is 'I cant remember' well I'm sorry but this was a major life event and I'm sure if it was me I would remember no matter how long ago it was. Maybe the truth is so bad that it can never be told?? It has taken this forum to make her see that because it was so long ago and because the lies have been going on for years that it makes it worse as its tainted every memory we have together.
You've made a determination to leave, and honestly... I do believe that no cheater is owed a second chance. For some people, infidelity is a deal-breaker, plain and simple. And it's actually best that, for people who understand this aspect of themselves, they go ahead and end the relationship. The most unhappy BS's are the ones who can't (or won't) recognize this in themselves.
But before you go, let me challenge your thinking on a couple of items...
You're very upset because you can't understand how something as serious as cheating could happen without the WS remembering every detail. But there's actually an answer for that... Trauma. The WS isn't traumatized by their actions the way we are. They might end up traumatized by the consequences and by the realization that they were not as good of a person as they thought they were, but the memories aren't processed through the slow-motion lens of a traumatic injury like our questions are.
If you ask me about any two boyfriends I had before marriage, I could tell you their names, whether we had sex, why we broke up, etc., but I couldn't give you details. I couldn't provide dates, or sexual positions, or remember conversations. Even important things that happened twenty-five years ago, I can't give details. I can't remember the last words my grandmother spoke to me before she died or an in depth conversation with my great-grandmother, and I loved them both dearly. Memory also varies from person to person. My sister has a much more detailed memory than mine. Our brains organize information a bit differently, even though we're genetically similar.
My point is that for the WS, their memories of past events are being processed in a very typical way. Some things stand out, others are forgotten, and there seems to be no more rhyme or reason to it than if I asked you what you ate for lunch twenty-five years ago. You might remember some places you ate frequently or what you liked on the menu, but the details are fuzzy. So, you are asking questions from a position of trauma. In your mind, these things were too important to forget. But WS's aren't traumatized by their own actions. These events aren't stored any differently than other events. In fact, if the WS is traumatized at the time of the cheating, it's even less likely that the memories would be clear, sharp, and in narrative order.
The other thing you've brought up a couple of times now is that your memories are "tainted". But that too is colored by trauma. I'm over five years out in a 35+ year marriage, and that was my reaction too, everything was tainted, nothing had meant enough to my WH that he wouldn't cheat, I had wasted my life on a worthless liar. But this too is a matter of perspective. You get a few years out from it, process the trauma, heal from the depression, and normal perspective returns. My life wasn't a waste. My memories have meaning and value. And while it's true that there were times my WH was pulling the wool over my eyes, for the vast majority of time, he wasn't.
Bottom line, trauma changes everything. But after awhile, the trauma gets resolved and you can see the world as it IS again. The memories you valued before, can (and probably will) be of value to you again. I know how hard that is to believe when you're in such a starkly bleak place in your life, but it gets better. Really, it does.
Without doubt, your WW robbed you of agency when she cheated before marriage. You might have decided not to marry her at all. And she doesn't have a time machine to go back and return that option to you. So, it's definitely within your purview to decide that it was a deal-breaker and opt to end the relationship. That said, the trauma WILL resolve itself at some point and these two factors you're citing above will not feel immediate and imperative the way they do right now. I just wanted to point that out to you in case you have any reservations.
For more in depth information on trauma and how it affects both your mind and your body, consider reading through a copy of The Body Keeps Score by Bessel van der Kolk. It explained so much of what I was going through, and I felt much better realizing that my trauma, and the way it had colored my outlook on life, was recoverable. I knew what steps I needed to take after that, and I could hang onto the knowledge that it was a temporary state of being.