I think you're going to do what you're going to do; however, if you could at least entertain the idea of not doing anything rash (like sending that detailed email to prevent yourself from going back), you might find strength in a blunt email with no details and be firm within yourself that you will not go back. This can be a true turning point for you, to abandon old ways of ending things by not burning a bridge.

I say this because I very much wanted to tell my previous therapist what I thought of her, but I realized that I would be giving her the gift of knowing what's on my mind. I wanted her to have the discomfort of wondering. It was a way to maintain my personal power and not let it leak away by trying to prove something to her.

It's just something to think about.