Let her in anyway. She might not leave, but if she does, you'll know that you gave yourself a chance to be your true 'dark' self with her. Your dark self may be treatable, even.
I don't know that I want to treat it - it's the core of me after all, and I've only ever felt "broken" in a marriage situation. And it's not like my thoughts are always dark anyway, it varies. It's more like a conversation between two people, with 100% trust, and where nothing is off limits - but it all takes place in my head. So it's difficult to share, because it's self contained and dynamic. As I mentioned, I grew up entirely by myself for much of the time, and I suppose my internal conversations grew to become my primary source of entertainment. It's hard to just "fix" that, it's who I am.
Given thoughts that are socially unacceptable in conversation, consider working them out at one step removed, as the thoughts of some fictional character in an invented story. Perhaps in the form of a novel, or stage play or film script.
