I'd discuss this with your current T; how long have you felt this way? Maybe something happened with your current T that triggered the feeling? I know that after someone dies or after an argument, we suddenly think of all the things we wish we had said.

I would write an email to her, since you have her address and she allowed email. I would not necessarily expect a reply (I remember the first time I terminated with my T because I moved too far away, I kept sending her poems and pictures and news (of my wedding), etc. but did not receive any reply. However, when I contacted her again, 9 years later, we got together again and worked together a second time.

But I would discuss the situation with your T, the desire to feel better yourself, perhaps at someone else's expense? If that T has "gotten over" your abrupt leaving and not returning, I think would be just as rude to bring it up again, so you can feel better, as it might have felt to not terminate gracefully in the first place. We don't get do-overs in life; we can only work with what we have, "now". Sometimes that means facing that we made mistakes or acted badly and learning to deal with that.