You said of the Affair Partner's Significant Other:
However, sadly he has done this multiple times and she forgave or pretended it wasn’t as big as it was.
Gently, like you are doing? If you saw a termite crawl across your kitchen floor, what would you know? That there are many more you have not seen. You just found a termite in your relationship.
The way you can tell if a newly caught wayward is lying is when their lips are moving.
For a reality check I recommend you read Mr.Flibble's thread here in JFO and then the followup in General. He is still learning what happened 1.5 years out and 9 months after divorce, and I don't think he knows yet. Of all of the stories on here, I was prone to believe his wife's story the most. Reading the feedback he got from some here was absolutely prophetic (some of the harsher voices, I might add). They nailed exactly what was going on.
I'm so sorry you are here. You must assume that EVERYTHING she says and does until absolutely proven otherwise is:
deceit,
subterfuge and misdirection,
seeking how much you know or suspect,
feeding you just enough to justify your concerns without incriminating herself more than absolutely necessary in this moment -
subject to change in the next moment,
causing you to doubt yourself and your instincts,
shaming you,
blaming you,
getting you to pity her,
distracting you,
changing the subject,
threatening you
and on, and on, and on, and on, and on.
And I would bet my mortgage that it is way, way worse than she will ever tell you.
This is going to take more strength and resolve than you thought possible.
I am sorry. But take heart, you have found a good place. Keep posting. Read the healing library and start the 180, for your own sanity.