So in the 2 years you've been together she's cheated on you TWICE, AND tried to hide a guy she was seeing once your relationship was open? To me, not only does she struggle with the concept of honesty, but there's NO way she's put in enough time to rebuild trust after breaking it that many times in that short a period and is in no position to dictate how long YOU have to decide if you can trust her again. Actions have consequences, and if she can't deal with the consequences of her actions in these cases that's her problem, not yours. It's up to HER to give you ample evidence that she can be trustworthy, and if she hasn't at this point that's on her, not you. If she had cheated, say, 5 years ago but everything had been fine since then and you were still mistrustful, I could sympathize with her giving you that ultimatum, but at this point I don't feel she has a leg to stand on. It may be that she actually wants out of the relationship but is manipulating the situation to make it seem like it's your decision to break-up and put the guilt of that on you while being able to play the "victim". But that's my own experience and bias talking and hopefully doesn't apply to your situation at all.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack