Its something you need to learn to use to your advantage. You can not change your physiology. You train for muscle memory so even if your "thinking" brain goes awol your lizard brain does what it needs to.
The amount of stress and adrenalin it takes to into fight or flight mode is both individual and increases with training and combat experience. I like to use my cousin as example because he is one of the best marksmen that I know. I would trust him to shoot an apple off my head at 25 yards with his pistol and a couple hundred yards with his rifle. You blow stuff up around him or shoot at him while hes doing it though and his accuracy is going to go to hell. Switching targets is going to slow down while his brain tries to process all the visual and auditory information. His brain is going to start shutting things out to try to cope with the situation. Lizard brain is going to take over panic and fight or flight.
You take an operator with thousands of hours of training and combat under the most stressful conditions imaginable. That person is used to jumping out of a plane at 40,000 ft, bullets flying, gun fire going off all around, explosions some controlled some not so controlled, the smell of blood and burning flesh on and on... That person is going to go through that situation with the same physiological conditions but a vastly different response. Your brain will still shut out a lot of the stuff going on as a matter of experience and focus on your mission. Identify threats, suppress threats, complete objectives catch a ride home, debrief, eat and go to sleep. For years later deal with PTSD...
To prepare for a SD situation IMO is to practice your draw and fire. I train people on 3 targets. left to right, bang, bang, bang bang, back to the left bang, bang. Do that until you could draw and fire at a man sized target at 20ft blind folded and you will be good to go. Practice the draw and dry fire! The drawing is as important as the shooting. If you mess it up it does not matter how well you shoot.
Sorry long winded this morning!!