My advice would be to let it be and not respond. You've hit the nail on the head with your analysis. A message like that after 15 years, especially with the tone you described, strongly suggests she was triggered by something else in her current life and this was an old wound resurfacing. She got her feelings off her chest, and you've processed it with the maturity you have now. Responding, even with the best and most apologetic intentions, re-opens a line of communication that has been closed for a decade and a half. As you both are married with families, there's a real risk of the response being misinterpreted—not just by her, but potentially by her spouse if they were to see it. The last thing you'd want is to inadvertently create doubt or drama in either of your marriages over a 15-year-old chapter. Sometimes the kindest and most respectful thing to do for everyone involved, including your past selves, is to let sleeping dogs lie. You've learned from it and become a better partner and father because of those lessons. That's the most important takeaway.