Do you think your SI is decided from birth or not?
I'm attached to SI (International System of Units), because I was raised in a country where this system is commonly used. Nearly nobody makes measurements in feet, miles, gallons, pounds, °F, etc there.
Speaking seriously, I think that explaining CTB failures through influence of some "survival instinct" is a pseudo-scientific BS. Avoidance of present or potential future discomfort is actually a way more generic thing than just fighting for life.
If you're a heavy smoker and you try to quit smoking right away, you'll likely face with severe withdrawal symptoms that with high probability will make you relapse due to unbearable ongoing discomfort. I guess, nobody explains such relapses as "my survival instinct kicked in, so I had to consume a yet another portion of a poisonous substance to stop my suffering". It would be ridiculous to associate the urge to smoke with the desire of the organism to fight for life. However should the same psychological mechanism, based on strong aversion towards discomfort, determine the failure of a CTB attempt, survival instinct or fight for life will be immediately blamed by most people here. This is probably because "survival instinct" sounds way more dramatic and therefore more appealing than "discomfort avoidance".
Sometimes people avoid some social interactions due to fear of rejection. Although rejection commonly doesn't pose any somewhat significant physical risk to health or life (similarly to the aforementioned withdrawal symptoms in smokers), the fear of it can be very strong, making the related social contacts remarkably challenging.
And the most interesting example of avoiding unpleasant experiences is the desire to commit suicide. The concept of survival instinct plain sucks at explaining how depression and suicidal thoughts are possible in a biological machine which is programmed to survive by any means. It just arbitrarily counts some traits of human psyche that are advantageous to maintaining the life as self-preservation instincts and ignores everything else that doesn't fit in it well. For me this approach looks no more scientific than explaining CTB failures through divine intervention which happens only when the god himself decides to interfere.
A broader view of how discomfort avoidance can influence our decisions may offer way more productive perspectives at finding the means of coping with the related psychological complications than constantly making emphasis on how "your organism makes anything to survive", which is basically nothing but a sort of defeatist whining.
In particular, the common remedy for fear of rejection is developing acceptance of possible rejection. Acceptance of the risks associated with a CTB attempt is likely one of the key factors determining the odds of chickening out during it. You may be 100% sure that you want to die, but if you haven't achieved some internal agreement with the taken risks, this can easily make you reconsider the need in going through a particular CTB procedure at the last moment.