These are things that have been accepted as truth because they have been said so often. The problem is that many of those repeating these things have no first hand knowledge and so they repeat what they have heard as if they knew firsthand. 
A myth repeated often enough becomes a widely held belief...almost a religious belief. yet, it is still a myth, and myths are not truth.
Myth Number OneThe presence of the gun is all you need, so have a gun. 
Having a gun is a good thing, but it is actually less important than your willingness to use it, your ability to use it well, and your understanding of the dynamics of the fight. The man with a gun who has no ability in its use, no understanding of the dynamics of the fight nor come to terms with using it to kill another man, will die with it still in the holster. Or worse, will over react from a fear-driven perspective.
Myth Number Two - It is all about shooting skills
Very few "shooting instructors" have any personal understanding of violence and so their entire world view consists of the artificial world of the shooting range. Their entire understanding and training revolve around the ability to conduct shooting drills and this is what they pass on to knowledge seeking students. 
Shooting skills have a value, and it is true, that only hits count. But the events that take place before shooting is of greater importance. As well, a proper reaction depends on your position in the fight, who possesses the initiative and your ability to exploit the gaps in the timing of the event. None of those things are learned on the shooting range.
Myth Number Three - "I was in fear of my life". 
All you need to do is utter those magic words, and the world will freeze in place. Like Dorothy and Toto, you will be instantly transported to your lawyer's office where he will save you with his lawyerly skills. And so those magic words and the magic phone number are all anyone worries about. They may even buy some insurance so they can feel better about not knowing things and be free from having to learn anything other than shooting drills. 
Well, those magic words never had any magic, and that magic phone number never stopped an investigation from proceeding. So rather than the myth of the magic words and the lawyer from the sky, it is better to know when, where, and why you are justified in killing an adversary, as well as how an investigation starts and develops so that you can make informed decisions and use the correct words to describe what transpired than sit, charged with a crime due to your silence, writing a hefty check you didn't need to write.
Myth Number Four - If you shoot someone, you will be arrested and sued. 
This myth is promulgated by those who sell legal services and associated products. Here is a secret - victims do not get arrested, only suspects do. 
Wouldn't it be nice to be the victim whose story was the main narrative in the report? The police have a goal. To conduct and conclude a thorough investigation in a timely manner and in a way that they can show was professional. They are not out to "pin a crime" on a victim.
As far as being sued, you don't need to shoot anyone for that. The American society has created an environment where every man or woman will likely be sued in their life by someone else. Wise people take steps to protect and conceal their assets.
Myth Number Five - You will get PTSD if you shot someone. 
I saw an ad recently. It began with the title - "The Worst Day Of Your Life", and lead into it being the day you defended yourself with a firearm (I assume they meant that you shot and killed some thug or terrorist who was going to do the same to you). 
I would think the worst day would be if you failed to shoot him because you left your gun at home or worse...missed your shot. But successfully putting down an evil man who sought your death or that of your people is The Best Day, not the worst one. It is a matter of perspective and self-image. If you allow such "Worst Day" thinking to become the norm for you, yes you will very likely have emotional issues. However, if you reject such thinking and have a more realistic self-image about such thing, your reaction will be quite different than the tear-spilling of the worst-day believers.
Myth Number Six - Physical fitness is not important because I have a gun. 
Truth be told, most gun people are in greater danger from diabetes and heart disease born of food abuse and obesity. Just go to a shooting range or gun store and look at the people there. What will happen to them when they get that sharp adrenaline spike, assuming their system is working properly, when all they have been doing is feeding their faces for the last twenty years? What will happen when they take a shot themselves as the opening shots to a big unexpected gunfight? Will their already taxed system be able to prevail and fight through (a factor of physical fitness), or will they succumb to a sudden heart attack from the shock? 
I will say that upwards of 80% of gun people would be better served taking a year off from shooting and spending that time at the gym lifting weights and reading about what they should and should not eat. Strong and fit people are harder to kill and are better at killing that weak and overweight people.