I suppose my understanding is that SI is a reflex type action. Most people wouldn't be able to stick their hand into fire and leave it there. I think when the body knows you're trying to kill it, it likely floods you with adrenalin to frighten you into stopping. For me- I think SI is more about the actual act of doing it and the body fighting back.
Whereas I think if you are having doubts about whether CTB is the right decision, I don't think it's so much to do with instinct. That's more cerebral to my mind. There can be plenty of reasons people think CTB isn't the right choice for them at that time. Maybe they still have a tiny bit of hope they could turn things around, maybe they don't want to upset their loved ones, maybe they're scared of what might happen if the attempt fails. I don't think any of that is to do with the natural instinct to try and survive though. I think that's more of a drawn out thought process weighing up the pro's and cons. When we act on instinct- I don't think we're actually thinking that much at all! It's more of a very strong drive. (To my mind.)