The real issue is that you care too much about information that doesn't affect you.

In this specific instance, the timeliness for when she dated her ex has no relevance to you whatsoever. So it doesn't matter what she tells you. Her words are just a silly little girl's game, and you treat it as such. You should act the same way as if your 10 year old sister told you the exact same thing.

Of course, it would be much different if she was cheating on you, or cheating on somebody else with you. Then you would be right to be upset.

Think about it like this. Would Jason Mamoa Leonardo DiCaprio care about one of his girls' dating timeliness? No! He would just bang them for awhile until he gets bored.

To fix your issue, you have to reframe the world first. Separate information that affects you vs. Info that doesnt.

Women don't care about facts man.

If you're dealing with one who doesn't have the humility to give a shit about being transparent the correct response is to terminate your relationship with her

Arguing does nothing. They will lie and gaslight you if you're not good enough at arguing and if you are too good at arguing they will shut down because their ability to weasel out of it is vastly outpaced by your instincts to blow past it

The correct response is to cordially correct them and ask about something of importance and if they give you substandard explanations you leave and don't tolerate that

At the same time I don't know why you made this specific question to her a big deal in the first place as you should have already vetted her story a long time ago and shouldn't have escalated it nearly this much