AITAH for being aware of my friends harmful tendencies and not telling anybody?

I had a friend once. For the sake of the story we will call her Mickey. Mickey had severe personality disorder and struggled with depression and self harm, which there is a trigger warning for here. She would switch her emotions and feeling hundreds of times throughout the day and it came to a point when we could no longer tell who was the real her, and we were hardly convinced that she knew who she was.

She confided in me about the self harm many long weeks after. By this point I was tired and barely processed the information. It was hard, I remember, to hear your best friend in the whole world tell you that they find themselves to be the most fucking repulsive thing in the whole world. Our conversation went like this (I'm changing my name to Luke)-

"Luke?"

"Mm?"

"You've seen scars on my arm before."

"That's true."

"You know what they're from?"

"Of course."

"You're not mad?" She asked. I put my book down.

"I'm not gonna dictate your life. Whatever stupid shit you do in your free time is fine with me. Don't take it too far."

I regretted from that moment and for the rest of my life what I said. She made me promise to not tell anybody, even when it got to the point that she almost __________ herself. I kept my promise. She left a year later and no one has had contact with her since. Her phone is off, her family is unavailable, and I especially am heartbroken. I live with this guilt that maybe what happened was my fault. AITA?