We are programmed to survive so we can pass on our genes. On a biological level, that's the whole point of our existence. To perpetuate ourselves beyond our deaths so that the species as a whole is more likely to survive.
The whole point of the survival instinct is to ensure that this happens. We need to be terrified of our deaths/terrified of any situation that could cause our deaths in order to survive long enough to procreate. The problem is that when/if we pass on our genes, the SI doesn't just switch off, it pretty much continues with the same intensity our whole lives.
I think there could be a more physical and a more mental/cerebral aspect to SI, though there is overlap. The physical one is the one that makes us afraid of (and therefore avoid) e.g. heights, spiders, snakes, and makes us avoid sources of pain on an almost pre-programmed level (e.g fire, sharp objects).
The more mental aspect is the sort of voice in the head which says e.g. "death is the unknown and you should be afraid of the unknown", "dying will cause you to suffer", "you may not succeed ctb and be worse off", "death may be worse than life if hell exists and you end up there".
I think both these aspects are subservient to the primary goal that I mentioned, passing on genes to perpetuate part of ourselves after physical death.
I would think it is almost impossible to switch it off simply by an act of will, but we can dull it maybe with certain mental techniques like repeating certain phrases to ourselves, even taking the methods of cognitive behavioral therapy and sort of subverting them to combat that mental voice which tells us how bad death is and how afraid of it we should be. Practicing mental visualization of whatever method we have chosen to ctb possibly helps, to accustom our mind to it on a more subconscious level.
I also suspect overcoming SI is unique to each individual, depends on circumstances, level of suffering/anguish etc.
These were just random thoughts and I'm maybe stating the blindingly obvious, but there may be something there to develop further