I feel you on how these convos can leave a bad taste in your mouth. She's having a need to explain herself to you so that she can get rid of her negative emotions - again, that's just burdening you with it and you don't deserve that.
I don't have a wicked response for how not to come across as cold in these convos. I think as much sincere validation you can give is the best - and feeling detached really helps making it come across as authentic. I'd say something like this:
"I know this difficult, but at this point with so much time passed, I really don't have any desire to discuss our past. Good luck on your journey."
I think softening the language can help. You get the message across but not be as cold. But again, you can try all the best you can, and they'd still interpret it as a cold. So, I wouldn't worry about it too much. Do the best you can to convey your thoughts in a manner that is calm and direct. I think that's good enough.
