Probably the biggest problem is a failure to react quickly enough.
A person within 21 feet of you can get close enough for a physical attack before you can draw, aim and fire a weapon. Therefore, don't let them get within 21 feet. Use verbal commands and back off if it is possible. Regardless, draw your weapon and go to a low ready position (weapon in both hands, pointed toward the ground immediately in front of your aggressor.
Once the weapon is drawn and in low ready, be prepared to come on target and fire while the aggressor is still outside, or just inside, the 21 foot circle. Be prepared to stop the aggressor before he can get close enough to attack you. Practice so that you can do it by muscle memory. Think through your response well in advance, i.e., weeks or months, of the need to employ it (kinda like diving).
Learn to shoot straight. Practice, practice and, when you're tired and bored, practice some more.
Be prepared to render first aid to an aggressor who is no longer a threat.
If you're involved in an incident, the only thing you need to tell the police is that you want to be taken to the hospital. Don't talk to them about the incident until after you've been evaluated, had some time to rest and retained counsel.