I half chickened out and sent her an email.
Dear _______, I was in your class more than a year ago. At the time, I was having mental health issues that I did not understand. LOL, but you probably figured out that much from our email exchange. Part of it was PTSD, and in one class there was a guy yelling at his dog. He scared my dog to pieces, and even got my PTSD going. After the class, I wrote you a nasty letter, which I have always regretted. I am sorry about that letter. You did nothing wrong and did not deserve that.
I am doing a lot better after some therapy and meds. I would like to try your class again. Would you have me back?
She has not responded. We'll see. I feel better at any rate.