I think you have to make a decision based on what you know of the situation OP, and what you feel comfortable with.
It's not really for your therapist to say that you'll feel eaten away at by things left unsaid. Especially not if you think the friend in this situation will be manipulative or twist what you say to her.
From experience with a family member, I would say that offering as little as possible by way of explanation is better than trying to go into detail.
If you were trying to save the friendship, maybe it would be better to raise whatever the issue is so you could both hopefully try to resolve things. But as you seem certain that you want to end it, what is the point in airing issues or grievances and then walking away anyway?
I spent far too much time trying to explain my hurt feelings to someone who didn't care and twisted everything I said.
I've found it much better to adopt the broken record technique now and just say things like "We have been through this before, if you don't understand now you never will" or "I have made my decision, you don't have to understand it but you do have to respect it."
Maybe that might be the way to go, if your ex-friend asks for an explanation, just say that you don't feel it will be helpful to go into everything, you feel that the friendship isn't working for you or has run its course, and you'd prefer to leave things without argument or accusations. Then repeat as necessary.