People are crazy and we all have our own issues.
It's just whether or not we're willing to try and help others with their issues that bind friendships together. It's true.
She's threatening to cut herself because she's either being an idiot or she's unstable. I must say we're all idiots from time to time and some of us crack easier than others, causing us to make stupid decisions like cutting ourselves.
Either the case, it does not matter.
What matters is if you feel it's necessary to put effort towards repairing your relationship with your friend and her issues or not.
Imagine if you were her.
Let's say you got a boyfriend and she yelled at him. Just picture your own friend yelling at your beloved boyfriend and when you ask why, your friend tells you she promised not to tell you.
So now you're conflicted between someone you love and someone you admire as a friend wondering why they can't get along.
I'm sure you'd be pretty desperate to find out exactly why there is a problem between them, and I'm sure she is feeling that way now.
With no other option to convince you to fess up, she's playing her last card.
Will she cut herself? Probably not.
But the fact that she even told you she would, should give you a good idea of how desperate she is for answers.
The fact that she now knows you are keeping a secret is reason enough for you to tell her what is going on.
It's fine to keep secrets, but telling someone you're keeping one from them is torture.
Naturally she is trying to figure out what those secrets are and unfortunately she's probably assuming the worst.
Which again, is why she's so desperate.
So don't let a "secret opinion" be the reason you end ANY relationship with anyone, ever.
You've already spilled the beans, so it's time to confess. It's only right.
Maybe she's not being the best of friends by threatening you with self harm.
But perhaps YOU can be a better friend than her by telling her what's really going on.
And if she doesn't accept what you have to say about her boyfriend, then so be it.
At least you'd end the friendship knowing you did the best you could to help her out, as her friend.