The way that I see it, I think that those who would be able to answer your question best are no longer existing here. But anyway, there are just no easy answers to this question, it's more like a feeling that one has where they know that it's time to exit this world. And once someone has a method that they feel confident in, then they can just exit when the time feels right.
Maybe many people who were successful in ctb reached a point of complete desperation and this allowed them to overcome the instinct to survive. But it's true that it's like the survival instinct exists just to prolong suffering and keep us trapped here, I do understand that it can be very awful and frustrating when you wish to be gone yet feel stuck in this world.