Okay so I know it's been a week since the Nancy Drew finale but I finally figured out how to verbalize my thoughts.
By the time Nancy ends things with Ace, she has to know about his insecurities regarding her. If nothing else, this conversation gives it all away:
I personally believe that this is also something that Nancy would have figured out in the AU where they were dating. You don't have to be with someone very long before you start noticing their insecurities, especially when you're someone as perceptive as Nancy.
Nancy couldn't go for the killing blow when she rejected him. She only ever told him he WAS good enough.
After the barometer breaks and Nancy realizes she has to leave, she could have easily used Ace's perceived failures against him. In fact, it's the only explanation he might have bought.
So through that lens, Nancy's act of cruelty in rejecting him moments later becomes an act of grace. She knows where his weak spot is, and she chooses not to use it. She knows she has to hurt him to keep him safe, but she can't bring herself to go for the ultimate hurt.
Alternatively (and this is my favorite theory), she is intentionally feeding Ace a line she knows he won't buy in hopes that he'll figure things out for himself. We know Nancy doesn't usually mince words for the sake of niceties, and this would be exactly the kind of loophole she would set up for herself. If she had told Ace he just wasn't good enough, he would have believed her and moved on. But now he suspects she's in trouble, and he won't stop until he figures it out.
