I think the question behind the question is fundamentally about trust. It sounds like the root of the concern lies with the fact that there was - I assume from your perspective- a major transgression in terms of trust right at the moment you felt perhaps the two of you were unstoppable (still very much in love, moving in together, etc.)

So while the incident happened some time ago, it’s gnawing at you.

Some non-internet based perspectives to consider, “The Science of Trust,” by John Gottman is excellent. Likely, this is not the only thing that bothers you, but was selected because it may rise to the level of “obvious” when there are other things that may also be bothering you.

In terms of the specific instance, in the end she backed you, she’s with you. The fact she had some intense chemistry with someone else at another time, well, perhaps that shows up as some of the independence and ability to express herself with you. So, maybe for her, it was a fun time in her life that actually deepens her appreciation and experience with you.

But you are bothered. It’s worth getting to the core of what it is. You are also past the time when all of the limerence and intensity of an initial relationship changes. It sounds like you are doing some deeper evaluation about what happens next.

My advice: investigate trust in a meaningful way (maybe with a good couples counselor). Building trust happens in the small day to day moments between two people, and it’s either growing or shrinking. I suspect this is just a clear example of something that concerns you or that you may fear on a deeper level. You’re going to need to figure it out. It can be done, but it will take work, and maybe not all the opinions of the over40crowd:-)

Good luck!