This is very delicate. You don’t want to spill all the beans, because it may make your new girl think you can’t keep a secret. Hint at a personality disorder, but nothing specific. You don’t want it to seem like you care TOO much about your ex’s mental health.
“She Was A Sweet Girl”
The ultimate backhanded compliment. Five words, all damning, with the optimal words being “was” and “sweet.” (“Was,” because your ex is in the past. And “sweet,” which is just a terrible way to describe someone — it’s how they describe gorillas who know sign language.)
“Her? Oh, I Dunno.”
As in, “What’s that ex of yours up to?” This makes you sound totally disconnected and disinterested, and reduces your ex to a face on a milk carton. Just make sure to delete the two hours of Facebook stalking from your browser history.