In the older thread, you said you told your old therapist and she said it was nothing she would have to report. In other words, you are not a current danger to anyone, nor was the offense so criminal that it needed to be reported in any way. Your new therapist will almost certainly have similar take on this.

It was, in relative terms, a minor offense, and most likely more of an awkward teenage blunder.

It seems odd that you have stayed so fixated on this one event now for over two decades. I would assume your therapist will reassure you that you are not a horrible person because of this one event, but she will also delve into why this one event seems to be driving the status of your mental health and will guess that this fixation on this one error is a symptom of something else going on.