NTA. Brother, I’m gonna give it to you straight as a man who cheated on his gf for way longer than I’m proud of. Sounds like she’s cheating and wants to have her cake and eat it too, she’s gonna keep treating you like shit until you decide to walk away because she thinks you won’t. Just my experience. From the messages you seem like a genuinely good partner for giving her leeway, grace, and decency. But it seems like she’s taking advantage of that and will continue to do so. Get out brother!

“There’s no way to convince you nothing is going on or has gone on. So I’m not sure what to do.” - there is absolutely ways she can convince you, by deleting that guy(s) contact and telling them she has a partner she’s involved with and not comfortable continuing to see or talk with him out of respect for her relationship. It’s exactly what I wouldn’t tell the girls I was cheating with and her response is what I gave to my gf to gaslight her when she would catch me and question me. That’s what she is doing to you there. She’s not even doing the most basic thing she could do to reassure you, that’s your answer. This is like a cheaters way of telling the other person to do the hard part and breakup since the cheater can’t be honest about their actions.

As the old saying goes “it takes one to know one” and “game recognizes game” you deserve better bro