There are a few things that need to be considered regarding the mental, emotional and psychological aspects of fighting. Specifically, we are talking about the role of ferocity - extreme aggression - in self-defense. Although an understanding of the importance of aggression in self-defense is one of the best attributes a person can have, it is also the most underdeveloped attribute in 99% of the self-defense enthusiasts I've met. Without thorough mental, emotional and psychological training, your chances of winning a fight diminish.
In my experience, most martial artists are not good fighters at all. Many spend their time learning techniques, with little or no time given to mental or emotional preparation. Few people are truly prepared for the intense mental and emotional stress of chaotic, "in-your-face" violence. They think they are prepared because they've studied martial arts and they know techniques. What they don't realize is that techniques often don't work as planned when the person is experiencing the extreme stress of an attack.
Begin by replacing the reactive term "self-defense" with the proactive term "fighting". This helps to get you into the right mindset. Your training routine should integrate full-contact sparring and role-playing exercises where you work through real-life simulations and scenarios. Reality-based training should be the cornerstone of your training routine.
As you train realistically, keep in mind that you must completely control the fight, rather than allow your opponent to control it. This is very important when it comes to winning a fight in real life. If your opponent/attacker is controlling the fight, then you are allowing him to force you to play the victim's role. You must reverse the situation through the use of aggression. Force the attacker to play the victim's role. Unless you train this way, you just have a bunch of techniques that you won't be able to use effectively in a real fight. Remember, any martial art and its techniques are just tools. You are the craftsperson who must use them effectively.
Extreme aggression can help make up for a lot of mistakes in a fight. It is extremely hard to defeat someone who is extremely aggressive, even if he hasn't studied martial arts at all. He just won't go down and you can't seem to hurt him. He exhibits no external indications of fear, just a completely overwhelming ferocity that is terrifying in its intensity.
This is how I fight. I don't want to have to "fight to the finish". The phase,"And may the best man win" is not acceptable to me. I want to win whether I'm the best man or not. If I'm not the best fighter, I'll cheat or terrorize to win. I will win at any cost.