You can’t answer an irrational question with a rational answer. You made a sincere, rational, and romantic response to her question. Most women would be deeply moved by it. But your girlfriend ins’t satisfied. That suggests that her “what if” question is probably not about your relationship. It’s about your girlfriend’s insecurity. I suspect that somebody in the past hurt her badly by reconnecting with an ex. You know that expression “Once burned, twice shy”? She’s now assuming the worst of you based on that prior experience.
I suggest you talk frankly and directly about why she is so afraid and see how the two of you can address that. Reassure her that all men are not alike, regardless of her experience so far. You might tell her that from my point of view, she should be more worried if you were the kind of guy who had to cut someone off completely to avoid being pulled back into a relationship. If you were that vulnerable, she would have far more to worry about. As it is, you are so done with your ex that you are certain you can stand up for your present relationship even if your ex texts you. That speaks well for your principles and for your commitment to your girlfriend. At 38, I’m guessing that you are done with playing games and are ready to make a life with the one you love. Focus on helping your girlfriend hear that.
I wish you well.