20 Years ago, I subscribed to the Time Life WWII book series and read every one cover to cover. I was a little older than draftee age at that time but still well within the age group that fought. The first one where I began to try and imagine how anyone could actually fight was the Battle of Tarawa. I tried to imagine wading through 500 yds of neck deep water weighted down with 100 lbs of equipment in the face of machnine gun, mortar and artillery fire.
Yes, a slight miscalculation of the tides at Tarawa had most of the vessels that were moving the men from the ships to the beach hanging up on the reef forcing the men to wade/swim all the way to the beach under withering fire from the island.
Can't really imagine it. I don't know if I could make my feet move, my knees bend - etc.
I was in combat only once (Vietnam). I can't really speak for you, but I suspect you would surprise yourself with what you could do. I know that in a matter of seconds. I had the following conversation with myself.
"HOLY ****!!!!!!! There are people out their trying to KILL ME. WTF! I'm a nice guy! I open doors for ladies! I squeeze the toothpaste from the end of the tube! Why me??? How come they want ME dead??? What can I do??? What can I POSSIBLY do to prevent them from KILLING ME???? First thing I gotta do is keep them out there, cuz if they get in here their odds of success increase dramatically. How do I keep them out there??? I'm guessing curling up in the fetal position and whimpering won't get it done. I GOT IT!!!!! I have to throw things at them..................LIKE BULLETS."
So, you sit there, wondering if your bowels are going to betray you, and you begin to do your job. You fall back on your training and just do it. You do it cuz that's the only thing that is going to save your little terrified white butt (assuming your butt was white and little to begin with, and is still white).
What I cannot fathom is doing that day in and day out for months, getting a few days/weeks off and then doing it again day after day for months, getting a few more days/weeks off and then doing it again...........to infinity. I don't know if I could stand the cumulative affect of that much terror and devastation. How the survivors of World War II stood it and didn't end up crazier than a loon is beyond me.