react to the situation, follow your procedures and training. no hesitation.
IMO that is why you should train rigorously and in as stressful of an environment as you can safely replicate.
Years of doing training in Flight Simulators has show me how necessary it is to be able to initiate an emergency response without thinking about it...while at the same time blocking out distractions and staying on top of your situational awareness.
Performance in an emergency is a skill that needs practice. I've watched countless videos of myself and co-pilot(s) handling different inflight emergencies and there is no substitute for repetition, adherence to procedures, and experience.
When it comes to pulling the trigger if the situation calls for it, no hesitation. if I didn't feel confident in that, i wouldn't carry or even consider carrying.
I'm not one of those "if i have to pull my gun, rounds will be forthcoming" type of guys. (i hate the phrase "clearing leather") Pulling out a weapon can be an "almost" last defense as a deterrent...shooting being the ultimate last defense (debatable). That is why situational awareness is so important.
Drill, Drill, Drill. Home invasion drills, simulated ATM situations, carjacking, escorting a "date" defense, and others depending on your job and environment. You have to take your drilling seriously though, sitting at the range and plinking is not the same. (although increased familiarity with your weapons and better accuracy is good) Videotaping your training is an underutilized technique. You'll see the good things you do and bad. I've noticed that at times I get tunnel vision on my "attacker". So I work on keeping my head on a swivel. Now it comes second nature to me and the difference is noticeable when I review films. Now I'm confident that if it happens in real life, I'll be less likely to make the same mistake again...but I won't be thinking about it, it comes naturally like a reflex.
The same thing goes for pulling the trigger. People have different "breaking points" when it comes to when they'll fire. Lord knows PBP and Creature just spend about 2k posts arguing about ALMOST this sort of thing and the only thing we've determined is that:
1. everyone has their own opinion to some extent or another
2. Creature brings nothing to any discussion except for his contrary attitute. JK
Figure out your "breaking points" as best you can and practice situations that meet and also fall short of those standards. I know that i've screwed up in practice and "shot" my assailant when he didn't have a weapon, and other times I didn't notice a 2nd threat and hesitated (resulting in my "death"). So I keep practicing and practicing, training myself to look at hands and not faces, head on a swivel, identify escape routes, etc. Each practice session i'll look over the video of and assess how it went. I generally have 3 cameras in use for these things. Wide angle elevated camcorder to give overall view, then I will wear one of my cycling helmets with a helmet-cam and my assailant will wear one as well.
I would cringe if i viewed some of my earlier videos, it was uuuugly. But you learn more from your mistakes.
Anyway, it took quite a while, but I'm finally at the point where I consistently follow my self defense procedures w/o thinking about them and when it calls for me to "shoot", I do so immediately. I'm confident in a "real life" situation I would revert to my training and perform as close to this as my elevated adrenaline would allow me.
I really think you should take as many weapons classes as you can. Close quarter, marksmanship, h2h combat, whatever you feel comfortable with. The more you practice the more second nature it will become to act under extreme stress and IMO that is one of the hardest obstacles to overcome in any defense situation.