Oh don't get me wrong... by the time it got to the letter part things had already spiraled out of control. I don't fault you at all for reading the letter as you had to save face based on how she reacted. Had you not, you would have been branded a delusional liar. The reason she broke down after you read is not because of the personal element of it, but because you completely reversed the tables and she is branded as the delusional liar.

The piece I am focused on is the build up - that I don't think past relationships should ever been brought up in a personal setting. It would have caught her off guard and in the same way your mind was racing so was hers - she did not think things through clearly and how much of an impact it could have on both of you. Her knee jerk was to deny/lie (unprofessionally) because that maintained the status quo in her mind of the professional boundaries. Had she been on the ball she could have preserved her professional image better by agreeing and maintaining the same viewpoint as you: not a big deal/amicable.

All in all, I would say, whats done is done and if you were my coworker/friend i'd buy ya a beer and tell ya not to worry.