Being proficient is more than just being able to hit the bullseye on a target.
Some people refer to it as pistol craft. Like a wood worker knows how to use his tools, a proficient shooter should know how to use a handgun.
You must be able to reload under pressure.
You must be able to quickly recover from a malfunction such as a double feed or a failure to fire.
You must be able to shoot while you are moving.
You must be able to do all of these things while you yourself are being shot at.
Finally, in the event you're injured, you must be able to do all of the above with one hand.
Like accuracy, the ability to do these things comes with practice.
Somethings are hard to practice (like doing things under fire), but you can come close. Practice with friends. Do your drill while they pummel with nerf darts or airsoft pellets. It's not what they use that matters. It's how well you can use your tool under difficult circumstances.
When you are faced with a self defense situation, you will need to determine how best to respond.
Once you decide that the use of your handgun is a proper response, you must be able to respond quickly, properly, and effectively.
That doesn't mean blasting away like they do in the movies.
It means being able to draw your handgun and put the sights on the bad guy. If you can do that effectively then there is a chance that he may turn tail and run.
If he doesn't run and won't comply with your instructions, then he now has the advantage. You can only act in a manner that's reasonable based upon his actions. If he has a gun, you probably don't have much choice. If he has a knife or a less lethal weapon then your response must be more measured.
If he's got a knife and advances on you, you're options will become limited very quickly.
On the other hand, if he's got a knife but just stands there, then firing your handgun wouldn't be a reasonable response. You can both stand there and stare at one another all day long but you can't shoot him.
No matter what the situation is, your ability to respond and how you respond will be 2nd guessed by everyone. You must be as prepared as possible so that you can act in a reasonable manner.