You are in an abusive relationship. Seek help, including how to exit smartly.

Ghost her. Act normal, prepare a way out, then leave a note saying “you are abusive. I am leaving. If you can’t handle it call [suicide hotline].”

She got him to stay once by cutting herself so she knows it works. He needs to move out while she's at work and leave her a note, block her and change his phone number so she can't contact him, and let her family know he's leaving and that she might be suicidal. He shouldn't let her continue to abuse him. That's all he needs to do. He's not responsible for his abuser.

The people who threaten suicide are the often the last people who would actually do it. They're also evil, manipulative arseholes.

What a psycho hope he gets away in one piece

Not all mental health conditions are the same and someone who has BPD is on a level that most are simply not equipped to handle. OP is far too young to fully grasp the extent of this particular diagnosis has and how there is nothing he can do to help her. She has severe abandonment issues that are not always grounded in any reality. She will push him away to test him and then lose it if he moves an inch. Threatening self harm is horrible for all involved, but if she really meant to to do it, she would have waited until OP was not in the house. I'm not being cruel, I spent too much time staying with someone with BPD. There is no cure and it doesn't get better. Sorry to say it, but for OP's benefit, he should go now. These texts seem unhinged because she is and it really doesn't get better

Call her family to be there when you tell her in private that you're leaving; if she has no family near, call the police

*Holds up lantern ala Bray Wyatt* Brother OP, RUN!!! *Blows out lantern*

She is ill, and will drain you if you stick around.

8 months and they are living together? Most of the time I say to each their own. But the threatening to unalive herself and trying to prove it by cutting is unhinged. She needs more help than OP can give her. We all have past trauma, but this... wow. I agree with the people who said OP is in an abusive relationship.

You ABSOLUTELY KNOW it's not normal for one partner to have to know where the other is at all times! & The screenshots of the text showed just how irrational she is. I was shocked to realize, from other people's comments, that she isn't in therapy! She must have been at one time or she wouldn't know she has BPD - or has she ever actually been diagnosed? But you have to get out of there & have no contact with her ever again. Make your arrangements for another place to live & for moving. Tell her you're breaking up because 1) You cannot be with someone who doesn't trust you & expects to know where you are every minute of the day. You have never given her any reason not to trust you & if somebody cheated on her in the past that has nothing to do with you. She should get professional help for dealing with past trauma & with her trust issues; 2) You can't be with somebody who threatens suicide as a way to control you. If she is having suicidal thoughts she should call a suicide hotline or go to an ER. If she decides she wants to commit suicide, that's strictly her decision & you will not blame yourself for it;; 3) You're not able to deal with or help someone who has BPD & can't be expected to. She needs to find professional mental health care. And, yes, it's going to be a miserable, horrible event, drana, hysterics, & the suicide threat. When that starts, call 911. (She needs more than a 72-hr hold - she needs to be hospitalized. If her parents are in the picture, let them know what's taken place . & Move out. With no further contact - block her. Warning: If she doesn't get stabilized with professional help, she may become your stalker or cause all sorts of other problems for you. Be prepared to get the police involved & find an appropriate attorney you can call if it comes to that. I'm very sorry she has this condition but you can't help her or improve the situation. You deserve better but you're the only one who can make that happen. I'm surprised this relationship ever started - after being around her a few times it should have been obvious that something wasn't right & you didn't want to be involved with her. And there's nothing in your post that says anything positive abt her, nor is there any mention of love or affection between the 2 of you. You want, & need, to be out, & I think you know it. Have courage & do what needs to be done, for your own sake.

The question is, does she have Bipolar or Borderline Personality Disorder? Because I'm intimately acquainted with Borderline and that's exactly what it's like. It's emotional dysregulation, paranoia...I feel for them both.