The contradiction that makes CTB so difficult
Part I
As anyone who has experienced even something as basic as a bad toothache will know, acute pain will cause the linguistic mind to close down and the ability to think completely dissolve, such that all that remains is the act of 'experiencing' the pain. Rational thought and any sense of 'being in control' is entirely absent, leaving nothing more than a desperate need for the pain to stop, or at least ease.
Precisely the same situation emerges as death approaches. In fact, dying could be defined as a 'progressive loss of control', until even the slightest residual sense of control has collapsed, at which point total unconditional surrender arises (ego death), and the dying individual endures the final steps to organic death, a journey that may be horribly prolonged courtesy of the medical establishment, or appear mercifully brief thanks to lapsing into coma.
Either way, the dying individual is entirely subject to the whims of Reality, with no ability whatsoever to influence / control the course of events, or the nature of their experience (they may for example regress to an infantile state, as with dying soldiers calling out for their mothers).
Part II
Anxiety is triggered whenever an individual realises they have lost control of some key aspect of their lives. And if that loss of control is due to some factor beyond their means to fix, then that anxiety may develop into fear. Whether fear actually surfaces largely depends on the nature of the underlying trigger - is it persistent, or transient; is it manageable; can it be safely ignored? In other words, to what extent is the perceived loss of control fundamental to well-being?
Full-on fear is triggered by a steadily diminishing sense of control, and it represents the very earliest stage of dying, as a side-effect of no longer fitting comfortably into your 'ecological niche'. That fear will evolve into abject terror once the loss of control has become absolute, and it's at that final awful point that the contradiction at the heart of CTB emerges, and makes the suicide option so difficult to implement ...
Part III
Suicide is after all about taking control of your own death. The trouble is - as summarised in Part I - death intrinsically involves the closing down of the linguistic mind, resulting in an absolute loss of conscious control. Or, to restate the contradiction more plainly - the suicidal individual is trying to control something whose prerequisite is to completely forfeit all sense of control.
It is when that contradiction is unrecognised that attempts at suicide are undermined, for with the departure of the rational 'controlling' mind due to overwhelming fear, all that's left is the organic brain and its primeval Survival Instinct (SI). And it's at the very instant the controlling mind has closed down that SI steps up to take control in its place, solely for the purpose of rescuing its own abode - the brain - from destruction (and to hell with whatever the linguistic mind was aspiring to - it's gone, and no longer in control).
And if SI should fail and in turn lose control (as might be the case when say, jumping from a burning building), then the fear that SI aimed to address will disintegrate into terror and panic, followed by death.
Part IV
So preparation for death, suicide or not, must accomodate abandoning even the slightest expectation of remaining in control, and instead entertain the very real possibility of abject terror. Still, such an outcome can sometimes be bypassed with sufficient stupefaction, either self-inflicted, or via some other circumstance (as arises with NDEs), else - as proposed by Buddhist meditative practice for example - by 'emptying the mind' and 'not grasping' (as most famously demonstrated by Thich Quang Duc's extraordinary self-immolation in 1963).
It seems then, that any individual approaching suicide with an expectation of somehow remaining 'in control' throughout, is almost certainly doomed to fail, simply because 'being in control' and end-stage dying are contradictory / mutually exclusive states, such that awareness of that contradiction, and developing ways to circumvent it, must be part of any plan to CTB.
)
