Personal experience. I haven't read any books on the topic, but this one that Hotora posted earlier looks interesting.
I trained Karate when I was a kid, I had these fantasies about what a fight was. I grew up in some pretty rough places and I've both seen and experienced my fair share of violence. I know about the theory of fight and flight (most important is actually freeze I'd say) from reading about it somewhere, but I can't remember where that was. Anyway, I used to always freeze up because what I thought in my mind was not how it was in reality. I was scared. I never ran from any fights or left anyone in need, I tried to do the right thing and I've got my face kicked in a few times, but I didn't really act either. Even when I started doing MMA, I could go and go in sparring, I took pride in my toughness, but I still froze up on the streets, somewhat. I was overthinking things, something deep down was telling me not to do anything out of self preservation. It bugged the hell out of me. As I got older, things started to change. I don't hang with any bad crowds anymore, but I helped a few people from getting seriously hurt. I started acting on things and I realised it's all in the mind. I realised that fighting for real is not some movie. You have to embrace the hurt, you have to accept it and not be afraid of it. You have to throw ego out the door. You're not some expert and don't expect to look cool either. Expect to get bloodied up and EMBRACE it (that's important). That way you will never be struck by fear or surprised. That's the mindset I was using sparring and that's the mindset that started to carry over. Things just started clicking and with all the shit I've seen it made sense. You wouldn't reason with a wild animal, you wouldn't overthink things, you wouldn't expect not to get messed up. If it was about to pounce, it's either you or it. That's the mindset you've got to have. With that in mind, take it out.
Hehe, sounds rough and I don't advocate real violence at all. Better to stay safe and out of trouble because so many bad things can happen, but that is the mentality you need to have if you want to act.
Sorry for the rant!