You don't.

When someone who’s everything to you is bent on walking down an all too familiar path of self destruction, how do you stop yourself from running to them and dragging them off? When every fibre in your being is screaming for them to stop, turn around and let go of their pain, how do you hold your silence? When all you want is to keep their nightmares away, when all you need is for them to be okay, how do you convince yourself to let them do as they will? All these goddamned questions. They’ve been ringing in my head for days, now. I can’t find the answers. I’m not even sure if I want to, anymore.

How do I mend something that wants to stay broken?
You don’t.
Oh. Right. Because it was never really mine to mend in the first place.