I imagine you cannot think too much about things or too much ahead yes? You have to isolate some parts of your brain and wall the certain thoughts off. Yet you cannot do it completely because you will become too lethargic to care about your safety?
How to dull your instincts just enough to not go mad but not enough to get yourself killed from carelessness that perhaps would arise from separating your psyche from the existential dread completely?
What I mean is that when you are huddled in the trench and things explode around everything screams run and panic but you must separate from that feeling yes while still preserving rational thought and some modicum of self preservation instincts?
Edit: This post isn’t wholly theoretical but part of a mental preparation for increasingly likely war. During which I will most likely volunteer
In times of crisis, you’re liable to find out what you’re really made of. For all of history, unassuming folk have been surprised to find that they are made of pretty stern stuff.
That’s not to say we all come out of it looking heroic. In order to do that, we must be fortunate enough to realise that just because the world can be cruel, doesn’t mean we need be.
I guess I’m so many words my answer is that you will have to trust in yourself a little that you’ll know how to look after yourself. The real trick is going to be looking after the ones around you who make survival so much easier in the long run.