Recently someone posted the question on FB: How do you conquer fear?
Since the question is so open ended I'm going to narrow it down some. IMO, when it comes to self-defense you don't want to conquer fear. You want to recognize it and utilize it. Fear does some amazing things to your body that can help you. On the other hand, if left at unreasonable levels it can control you, over-exaggerating your responses—in any situation—not necessarily strictly self-defense.
Your body responds in two different ways—psychological and physiological. You need the psychological to drive the physiological. As such, it's the psychological component that needs tempering in order to make the physiological of greatest value.
Here are some ways your body responds physiologically: increased perspiration, heart rate and respiratory rates, glucose fuel dump for the muscles and brain, pupil dilation (allowing more light into the eye), increased muscle tone, decreased blood flow to the skin, intestine and kidneys (through constriction of blood vessels and veins), increased pressure to empty bowel and bladder, non-essential systems temporarily shut-down such as digestion and immune systems, and fine motor skills and thinking go away. Some of these responses can be lumped into what I call the adrenaline dump.
If you have a complete disregard for fear (meaning: if you pretend to have no fear) you can be just as likely to do something stupid as if you have too much fear. Both extremes are not operationally the best in many cases. However, there is exception, in those situations where the fear processes are over-ridden by preservation instinct, such as in a mother protecting their child or another human being helping another human being as examples. This temporary override is called Aphobia and is caused by a neuro-chemical dump. In some instances it is also possible for your body to not feel fear or pain—a state called Analgesia which can be seen in battle sometimes—an example is when someone gets stabbed and they say that the felt the pressure but they didn't feel any pain until after the fight was over. Unfortunately, there is no congenital fearlessness—so in that regard—everyone feels fear. Additionally, Aphobia and Analgesia are not something you can count on.
Let's look at what fear is from a big picture sense—at least in part—and I'm not an expert in this btw. Fear is your bodies self-preservation response to negative stimuli. The greater the perceived threat, the greater the body's response.
A lack of instinctual response may really mean that you did not perceive the threat accurately or clearly leaving you with an imbalanced perception of the situation. Likewise, too much instinctual response may mean that your perception of the threat is again, imbalanced. Imbalance in either direction is a more negative trait than having a rational amount of fear.
With that in mind, it is important to embrace fear. Which I think is what the previous posters where getting at. However, understanding what is good about fear and what is bad about fear may help more than a few quick statements.
Interestingly, my nine-year-old daughter is teaching me about fear management. Recently she went to a county fair with some friends of ours and she was afraid of some of the rides but she determined that the best course was to—and I want to user her own words—"I'm going to face my fears." And she climbed on these rides and did it. Without prompting. Not only once, but to really do it, she went on them each twice. Again, this is all without prompting. I was super proud of her strength and personal determination but also her very clear understanding of balancing the emotions, sensations, and instinct of flight, with some reasoned process for making it through the situation.
Anyone can do it. A process that can help is learning to meditate and associate stressor words with relaxation. This is a component of personal combat that I am really looking at. Imagery of stressful situations while practicing relaxing personal processes can help. It does not have to be religious meditation—I don't do that for myself. Instead, I use guided imagery and repetitive phrases, key words, etc. It's potentially very powerful by itself. However, I've also seen real value in applying the process physically in addition.
Hope that makes sense. There's more to this that I'm still learning and exploring but since it's been FOREVER since I've posted here I thought I would throw this out there.