I'm fairly sure that your to-be ex-partner probably isn't actively trying to manipulate you but it is manipulative regardless to have breakdowns and meltdowns when people no longer want to associate with you, and I am saying this as someone who has regrettably done this in my past. It's understandable, especially since it sounds like your partner was largely rejected by most of the people in their life and really doesn't have a social support group to lean on, but that doesn't change that it's manipulative. However, it is probably true for her that she isn't going to have a safe person for a while, and if that's what they need to feel safe then that means they're going to have to still find a way to exist with that lack of safety. Whether or not they recover will probably depend on their other connections or other hobbies. How long were you in a relationship for? If it was only like a year or two I think it's probably more likely that she'll recover than if it's 10+. Do you live together?

In my case, as the person who was regrettably on the other end of things (though most recently in the context of a friendship and not a romantic relationship), I don't think that there's any way to end any sort of relationship in the way of "I no longer wish to associate with you" that won't deeply hurt someone who is already chronically socially deprived and lonely. You are going to hurt them, probably deeply and in a way that will take a very long time for them to recover from, but if you stay in the relationship when you don't want to be, you'll hurt both yourself and them and you'll eventually hurt them even worse when you do leave. There's no way around or out of hurting them, particularly if they do not have good social support, which will also make things worse in the aftermath if they do not have good social support, but that isn't your responsibility even if they are entirely alone. You probably should cut contact entirely but I don't know that I can get the full context of the relationship with this brief explanation.